This study numerically investigated the performance of micro nozzle/diffuser pump subject to the influence of frequency, opening angle, and amplitude. It is found that the net flowrate of a micro-pump increased with pumping frequency and opening angle. However, a level off phenomenon of the net flowrate vs. amplitude is seen at an amplitude nearby 150∼200 μm and at an opening angle above 10 degree. This phenomenon is associated with two factors that compensate with each other. One is the free jet flow from the outlet that overturns and blocks the flow from the inlet. The other is the reduction of the strength of jet flow at a larger amplitude owing to effective increase of cross sectional area.