We report a MEMS device for continuous and binary separation of leukocytes (white blood cells, WBCs) and erythrocytes (red blood cells, RBCs) by their sizes. The system consists of PDMS cylindrical pillars inside fluidic chamber on glass slides. A 420 μm lateral separation was achieved for 5 μm and 10 μm beads. The critical particle size for separation was successful predicted by simulation and experimentally determined by calibration with polystyrene beads. The separation function (lateral displacement versus particle size) was also measured experimentally. Diluted whole blood and blood fraction of concentrated leukocytes were tested with the devices with erythrocyte to leukocyte ratio in agreement with blood count results. Experiments with fluorescent labeled leukocytes confirm the separation of leukocytes from erythrocytes and measured the separation efficiency.