A planar three-rhombus micromixer with two constriction elements for good mixing more than 84% at Re ≥ 20 has been demonstrated by simulations and experiments. Higher constriction elements with low blockage ratios may enhance significant fluid mixing by combining principles of focusing/diverging, recirculation and Dean vortices. The local high flow velocity induced by the high constriction element provides both high inertial forces and centrifugal forces for enhancing mixing efficiency under asymmetrical flow. Recirculations and Dean vortices are strongly influenced by blockage ratios and Reynolds numbers. The smaller blockage ratio and higher Reynolds number resulted in higher mixing efficiency. In simulation, the 84% mixing efficiency was achieved at the blockage ratios of 1/8 and Re = 20 together with a low pressure drop about 3630 Pa. The trend of the verified experimental result is in good agreement with the simulation result. A good mixing efficiency can be achieved using this simple micromixer with less mixing units at lower Reynolds number and pressure drop compared to the conventional chaotic micromixers.