This thesis addresses interactions between monetary and fiscal policies in a theoretical dynamic stochastic general equilibrium (DSGE) model of a small open economy and in an empirical model under a structural vector error correction model (SVECM). The thesis consists of three essays. The contribution is both theoretical and empirical that enables a better understanding of the complexity of interactions between monetary and fiscal policies in small open economies. The first essay examines the equilibrium determinacy under monetary and fiscal rules. The goal is to investigate how monetary and fiscal policy interactions ensure a unique and non-explosive (determinate) equilibrium for a small open economy. The study focuses when policy makers implement a set of policy mixes to address domestic output price inflation control for monetary policy, debt stabilization for fiscal policy, and joint output stabilization tasks. The result indicates that two policy schemes facilitate a determinate equilibrium. First, monetary policy actively controls inflation when fiscal policy sets a sufficient feedback on debt. Second, monetary policy becomes passive against inflation when fiscal policy is insolvent. Adding output stabilization to each rule simply causes variants of this fundamental. An interest rate rule with output stabilization can be more passive against inflation while providing a stronger response to the output gap. Fiscal policy is required to set higher feedback on debt along with its stronger counter-cyclical policy. The second essay links between the equilibrium determinacy and policy optimization. This essay provides insights into the design of policy mixes and compares determinacy outcomes between two theoretical models of a small open economy: with and without an explicit exchange rate role. This study shows that policy interactions in a small open economy with an endogenous exchange rate is quite sophisticated, especially when a monetary rule is added with an output stabilization task and/or targeted to Consumer Price Index (CPI) inflation. Additional concern for monetary policy in an open economy causes a partial offset to its reaction on domestic output price inflation that weakens its effect on the real debt burden. To minimize economic fluctuations, policy makers should mute the role of output stabilization for monetary policy, and set minimum feedback on debt that is compatible with the degree of counter-cyclical fiscal policy. Substantially active response to inflation is satisfactory for monetary policy with CPI inflation targeting. The third essay empirically presents monetary and fiscal policy interactions in Thailand's SVECM suggested by a theoretical DSGE model developed from the previous essays. This essay shows that the DSGE-SVECM model can be supported by Thai data. A shock to monetary policy is effective with a lag. Government spending policy is also effective with a lag and some crowding-out effects on output. An adverse shock in tax policy unexpectedly stimulates the economy, indicating room for enhancing economic growth by relaxing revenue constraint. Monetary policy is mainly implemented to correct a consequence of a fiscal shock on inflation (and also the domestic and foreign shocks), while fiscal policy appears to counter a consequence of the monetary policy shock on output.