Movable thin film membrane is one of the crucial part in a thermal actuated micropump development that acts as actuator to control the fluid flow. The working principle of this device is due to pressure exchange produced by thermal expansion of air in an isolated chamber. In this paper a deformation analysis of actuator membrane deposited on silicon substrate using Finite Element Analysis (FEM) is reported. The analysis is aimed to study the mechanical and physical behavior of the actuator structure. Parameters such as shape, size, dimension, material and thickness of the membrane are studied to find the optimum design. Correlation between the pressure and membrane structure are also observed. Simulation results show that circular membrane shape gives the largest deflection due to the lowest stress on its edges compared with other shapes of membrane at the same pressure. It is also found that membrane deformation decreases with the increasing of the thickness. However, only suitable deformation will be chosen for the pump application due to the limited chamber space. Comparisons at four types of membrane materials in this analysis indicate that polyimide has the largest deflection. Furthermore, polyimide shows the best capability to handle very hot temperatures since its melting point is very high. It is also elastic, robust and easy to fabricate. From the results, an appropriate membrane parameter selection are important in designing the thermal actuated micropump.