Due to its simplicity, the valveless pulsejet may be an ideal low-cost micro-propulsion system. Although a considerable amount of work has been done on developing different types of valveless pulsejets, understanding of the operation of these jets is still very limited. In this paper, computational methods are used to investigate how modification of the inlet size and placing an obstruction in the combustion chamber affect the overall performance of a valveless pulsejet in both a static and convective free stream. Different inlet lengths and diameters are simulated to optimize the geometry to yield largest net thrust. A 0.12 volume ratio between the inlet and combustion chamber is shown to be the best for a static pulsejet. However, this ratio must be decreased as the free stream velocity increases to account for ram compression. Different interior obstructions are also placed in the combustion chamber to modify the flow pattern and increase thrust. Results indicate that a properly placed obstruction in the pulsejet can significantly improve its performance.