In this paper, we exhibit a novel asymmetric bifurcating microchannel for cell separation by size. As developing this microchannel, we proposed the design and optimization criteria, which included the design of inlet and outlet based on the analysis of the Segre-Silberberg effect and the optimization principle to render the migration force at the bifurcation portion as great as possible. Experiments using rat fibroblasts and red blood cells verified these design principles and confirmed the separation function with the optimal design. The simple configuration of the microchannel makes it feasible to serialize several such microchannels within 1.0×1.0 mm2 space and to achieve enrichment rate as high as 103∼104 folds.