A valveless micropump was realized with a diffuser/nozzle shaped channel and a variable volume actuator which produces an oscillating flow. One-way flow may be realized in the nozzle direction since the pressure loss in a nozzle channel is lower than that in a diffuser channel. Pump characteristics were measured for various angles of a diffuser/nozzle element and positions of the actuator to investigate the effect of pump geometry on characteristics. The experimental results showed an optimal diffuser/nozzle angle for pump efficiency, and the optimal actuator position. The frequency characteristics and the pump characteristics were measured. Dimensionless variables were introduced to rearrange the measured data and to understand the physical mechanisms of the micropump.