Rapid mixing is necessary in the fields of the biochemical analysis and drug delivery. In this paper, a planar micromixer featuring the rhombic microchannel had been successfully demonstrated by the CFD-ACE simulations and mixing experiments. Rhombic microchannel will produce the recirculation to increase the mixing efficiency. The fluid mixing is related to the geometry parameters and Reynolds number of the rhombic micromixer. The better fluid mixing is attributed to the smaller turning angle (α), the more rhombi and the higher flow rates. From the design aided by the simulations, the three-rhombus mixer featuring the angle of α = 30° and a converging-diverging element exhibits good mixing efficiency more than 90 %. From the experiment results of the adaptive design, nearly uniform mixing had been achieved at Re = 200. Because of the rapid and good mixing, this micromixer can be applied to the fields in need of rapid mixing, such as drug discovery and protein folding analysis.