In this paper, the pumping performance of a piezoelectric micropump is simulated with commercial finite element analysis (FEA) software COMSOL Multiphysics 3.2a. The micropump is composed of a 4-layer piezo-composite actuator (LIPCA), a polydimethylsiloxane (PDMS) pump chamber, and two diffusers. The piezoelectric domain, structural domain and fluid domain are coupled in the simulation. Water flow rates are numerically predicted for geometric parameters of the micropump. Based on this study, the micropump is optimally designed to obtain its better pumping performance.