This paper numerically studies the performance of a piezoelectrically actuated valveless micropump with consideration of the three-way electro-mechanical-fluid couplings. Simulation of the piezoelectrically actuated valveless micropump (PVAM) indicates that both the pumping rate and the membrane deflection amplitude increase with the increase of the actuating frequency in a low frequency range (<7.5 kHz). However, because of the electro-mechanical-fluid couplings, the membrane deflects in an undesirable way at high frequencies. This will lower the pumping rate at high frequencies (>7.5 kHz). At even higher frequencies (>50 kHz), the pumping rate decreases further because the deflection amplitude decreases. This agrees with reported experimental results. The changing membrane deflection shapes at various frequencies clearly play an important role in the performance of the pump.