A valve-less micropump was fabricated by an asymmetric Y-shaped channel and a diaphragm. Numerical simulations are conducted to investigate the mechanisms of developed valve-less micro-pumps by using commercial software CFX-5. The numerical results show that the vortex generates near the diaphragm 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 comparison between calculated and PIV measured velocity profiles, it was found that the vortexes work as the dynamic valves and play a key role for the micropump.