In this study we have developed a valve-less micro-pump with one diffuser shaped element and a chamber with a diaphragm; the vibration of which produces an oscillating flow. The pressure-loss in a nozzle is lower than that in a diffuser, and therefore one-way flow is realized in the nozzle direction, The frequency characteristics and the pump characteristics are measured. The maximum total pump head and volumetric flow rate are 0.8 kPa and 2.4 ml/min respectively. The effect of working-fluid viscosity on pump characteristics is also discussed using water and glycerin-water solutions. As a result, the pump performances were found to decrease with increasing fluid viscosity and the pump performance depended on the Reynolds number of oscillating flow. The experimental results are discussed using a simplified model based on the Bernoulli's theory for unsteady flow in pump.