A valve-less micropump was fabricated by an asymmetric Y-shaped channel and a diaphragm. The measured maximum total head rise and volumetric flow rate were 600 Pa at zero flow rate and 1.3ml/min at zero total head rise, respectively. The velocity distribution in the micropump was measured using particle image velocimeter (PIV) to investigate pumping mechanisms for the improvements of pump performance. The results show that the vortex generates near the branch when the working fluid is supplied. And the vortex was found to appear at the inlet of the asymmetric channel in pump mode. As the result of the present study, it was found that the vortexes work as the dynamic valves and play a key role for the micropump.