Cardiovascular disease is the leading cause death in the United States. Although emergency surgery and medicine can be used for cardiovascular disease treatment, the survivors will suffer for it afterwards. Ultrasound blood flow measurement provides a noninvasive way for cardiovascular diseases diagnosis. In this dissertation, three algorithms for blood flow velocity measurements were investigated and optimized. Feature tracking has been previously proposed for vector velocity measurement. In Chapter 3, the optimal amplitude and time thresholds for feature extraction were investigated to minimize flow estimate variance while providing sufficient spatial and temporal coverage of flow area. A new method of lateral blood flow velocity measurement was investigated in Chapter 4 using the observation that the speckle pattern corresponding to blood reflectors stretches if the blood is moving in the same direction as the electronically-controlled transducer line selection in a 2-D image. The results demonstrated that there is a linear relationship between the speckle size and blood flow velocity, which can be used for ultrasound blood flow velocity measurement. To improve the lateral blood flow estimation performance using speckle size estimation, the relationship between blood flow velocity estimation for flow purely lateral to the ultrasound beam and flow gradient, random scatterer movement and ROI size was investigated and quantitatively assessed in Chapter 5. By changing the ROI sizes, the results showed that different optimal ROI sizes exist in different flow profile regarding the flow gradient and random scatterer movement. The method of blood flow velocity estimation using speckle size estimation was further tested in vivo with jugular vein blood flow from human subjects in Chapter 6. The results showed that speckle size estimation has a comparable estimation performance to spectral Doppler with potential time savings. Finally, a two-dimensional blood flow velocity estimation algorithm using apparent speckle pattern angle was proposed in Chapter 7. The apparent angle of speckle pattern changes with different scan velocities due to mis-registration between the ultrasound beam and scatterers. Results showed that this algorithm can resolve the amplitude and angle of the blood flow simultaneously.