Current clinical methods for the separation of whole blood into blood cells and cell-free plasma are currently based on large facility equipment, such as centrifuges. The disadvantage of this process is that the patients must have assays performed at the hospital or laboratory where the separation facility is located. The present study presents a design for microfluidic chips with different microchannel structures, which utilizes backward facing step geometry and centrifugal force to extract the cell-free plasma from whole blood samples at the branch of the microchannel for further assay, avoiding the influence of blood cells. Numerical simulation was performed on a personal computer to analyze the effects of inlet velocity and the structures of the microchannel on the flow field and back-flow in the microchannel, as well as the efficiency of separation and the volumetric fraction of the flowrate of plasma extraction. The minimum radius of particles (R) that can be excluded from the side channel, and fraction of the volumetric flowrate were obtained to evaluate the efficiency of plasma extraction. Based on the numerical simulations, the design with both converging and bending channels was the best design among the four layouts proposed. In this design, the value of R could be set to less than the critical value (set as 1 mu m because of the radius of platelets), and the volumetric fraction of the extraction flowrate was approximately 8.4% when Re was about 20. The preliminary experiments indicated the fluorescent particles with 2.5 mu m in radius were successfully excluded from side (plasma outlet) channel of the microfluidic chip with converging a inlet channel and the bent microchannel, when the Reynolds number of the inlet flowrate equals 50.