Valveless piezoelectric micropumps are in wide practical use due to their ability to conduct particles with absence of interior moving mechanical parts. In this paper, using finite element method, an extended numerical study on fluid flow through micropump chamber and diffuser valves is conducted to find out the optimum working conditions of micropump. In order to obtain maximum generality of the reported results, an analytical study along with a dimensional analysis is presented primarily, to investigate the main dimensionless groups of parameters affecting the micropump net flux. Consequently, the parameters appeared in the main dimensionless groups have been changed in order to understand how the pump rectification efficiency, defining as the ratio of micropump net flux to sum of the absolute values of fluxes of inlet and outlet valves, and optimum diffuser angle depend on these parameters. At last, a set of characteristic curves are constructed which show these dependencies. The optimum working condition of micropump can be clearly found out through the use of these general characteristic curves.