The objective of this study is to gain fundamental knowledge on the interactions between nanoparticles to achieve a uniform dispersion of nanoparticles in metals for manufacturing metal matrix nanocomposites (MMNCs). MMNC, also known as nanoparticles reinforced metal, is an emerging class of materials exhibiting unusual mechanical, physical, and chemical properties. However, a lack of fundamental knowledge and technology on how to achieve a uniform nanoparticle dispersion in MMNCs has hindered the rapid development of the MMNC field. In this dissertation, several methods were explored to achieve a uniform nanoparticle dispersion in MMNCs. In-situ oxidation method were applied to fabricate Al-Al2O3 nanocomposites with a uniform dispersion of Al2O3 nanoparticles. Pure Al nanoparticles were cold compressed in a steel mold and then melted in an alumina container. Al2O3 nanoparticles were in situ synthesized through the oxidation of the Al nanoparticle surfaces to form bulk Al nanocomposites during the process. Although some Al2O3 nanoparticles were distributed along the grain boundaries of some coarse Al grains, most Al2O3 nanoparticles were evenly distributed inside ultrafine Al grains to effectively restrict their grain growth. Moreover, the microhardness of the bulk Al nanocomposites is enhanced up to about three times as high as that of pure bulk Al. Friction stir processing (FSP) were combined with semi-solid mixing to disperse 6 vol.% SiC nanoparticles in Mg6Zn. Semi-solid mixing was effective to incorporate SiC nanoparticle into Mg6Zn matrix before FSP. The low temperature at the semi-solid state reduced nanoparticles burning and oxidation effectively, while a high viscosity of the metal at semi-solid state trapped the nanoparticles inside the matrix metal. Also, FSP was used to process Mg + 6vol.% HA nanocomposites with a uniform dispersion and distribution of nanoparticles after mechanical stirring. The mechanical properties of Mg nanocomposites after FSP were significantly improved. Unfortunately these two methods discussed above are not economical for mass manufacturing of MMNCs, while solidification processing is very promising as a versatile mass manufacturing method for production of bulk MMNC parts with complex geometry and high nanoparticle loading. However, the incorporation and de-agglomeration of nanoparticles in liquid metals are extremely difficult. Thus there is a strong need to fully understand the physics of the interactions between nanoparticles inside metal melts in order to develop new pathways to achieve the uniform dispersion of nanoparticles for mass solidification processing of bulk MMNCs. A theoretical model was successfully established to reveal the essential conditions for nanoparticle dispersion in molten metal during solidification nanoprocessing of bulk MMNCs. The interactions between nanoparticles in a molten metal include three key potentials, the interfacial energy barrier at a short range (1~2 atomic layers) to resist nanoparticles to come further into atomic contact, the attractive van der Waals potential (dominant in the longer range from 0.4~10 nm), and the Brownian potential, kT. Three possible scenarios for nanoparticles in molten metals were theoretically predicted below. 1. Clusters: when the maximum interfacial energy barrier is less than about 10kT due to a poor wetting between nanoparticles and metal melt, the nanoparticles will come close into atomic contact to form larger clusters in the liquid metal. 2. Pseudo-dispersion: If the maximum interfacial energy barrier is high enough (e.g. more than 10 kT) due to a good wetting between the nanoparticle and the molten metal and the van der Waals attraction is much larger than the Brownian potential, nanoparticles will be trapped into a local minimum potential to form pseudo-dispersion domains where dense nanoparticles are separated by only a few layers of metal atoms. 3. Self-dispersion: When the maximum interfacial energy barrier is high and the van der Waals attraction is smaller than the Brownian potential, nanoparticles will move freely inside the molten metal in a self-dispersion and self-stabilization mode. Based on theoretic study and availability of nanoparticles in the market, two material combinations, TiC (with a radius of 25 nm) in liquid Al and SiC (with a radius of 30 nm) in liquid Mg, were first selected for the experimental study. To avoid oxidation and burning of TiC nanoparticles, a novel method of salt assisted nanoparticles incorporation was developed to fabricate master Al-9vol.% TiC nanocomposites. A droplet casting method was developed to avoid the nanoparticle settling down and pushing during solidification. Microstructure studies revealed that TiC nanoparticles still form domains in Al matrix, indicating a pseudo-dispersion of TiC (50 nm in diameter) in pure liquid Al. However, TiC nanoparticles were successfully dispersed in the Mg18Al eutectic alloy. Mg6Zn-1vol.% SiC nanocomposite ingots were first obtained by ultrasonic-assisted solidification processing. A new method was developed to concentrate SiC nanoparticles by evaporating Mg and Zn away from the Mg6Zn-1vol.%SiC ingots at 6 torr in a vacuum furnace. After evaporation and a slow cooling at approximately 0.23 K/s, a sample with about 14 vol.% SiC nanoparticles was obtained in an Mg2Zn matrix. Material characterizations by SEM, EDS, and Vickers hardness measurements revealed that SiC nanoparticles were self-dispersed in Mg. Micropillar compression tests showed that the Mg2Zn-14vol.% SiC nanocomposites yield at a significantly higher strength of about 410 MPa with a good plasticity, while of 50 MPa with a very poor plasticity for pure Mg2Zn. In summary, this dissertation establishes a theoretical framework and developed experimental methodologies to achieve a uniform dispersion of dense nanoparticles in metals. The study has significantly advanced the fundamental understanding on the interactions between nanoparticles in molten metals to obtain MMNCs with a uniform dispersion of dense nanoparticles for widespread applications.