Micro mixing is very important in some micro fluid systems. Common design of micromixers is the multi-layer microchannel together with the complicated fabrication process. In this study, the planar design and simple mixing structure were adopted. This obstacle micromixer had been successfully demonstrated by the CFD-ACE simulations and mixing experiments. At low Reynolds number, high mixing efficiency and low pressure drop cab be obtained. 93% mixing enhanced by vortices had been achieved at Re = 20. Experimental results also show good mixing efficiency at Re = 20. Because of high mixing efficiency and low pressure drop, this planar micromixer is easier to integrate with other micro devices in micro-total analysis systems.