Being as a functional material, piezoelectric ceramics is applied to drive a micropump when it is composed to be a thin membrane on a brass film to form a diaphragm. According to the properties of the piezoelectric material, the stiffness of the diaphragm influences the vibration displacement and therefore the supply flow rate of the micropump. This paper focuses on the dynamic characteristic study of the micropump and the stiffness influence of the diaphragm on the characteristics. The mathematical model of the valve-less micropump considering the diaphragm stiffness is developed in order to predict the dynamic characteristics of the piezoelectric valve-less micropump. Using the 3-D finite element analysis (FEA) method, the static and vibration mode of the diaphragm are analyzed to obtain the diaphragm stiffness and natural frequency. The method using Matlab is to predict the pressure and flow rate characteristics of the micropump when the diaphragm stiffness is considered or not. Comparison of the simulation results shows the stiffness of the diaphragm influences the dynamic characteristics of micropumps. It also shows that the method combining FEA software with Matlab provides a possible and effective tool to take the diaphragm stiffness into account when the pressure and flow rate characteristics of micropumps are studied.