A design of a microchannel for continuous separation of particles based on alternating current dielectrophoretic is studied. The geometric design parameters are determined by analyzing the dielectrophoresis force field inside the microchannel corresponding to the most efficient separation. The trajectories of 5 and 10 microm spherical particles are derived analytically using Lagrangian tracking method to examine the feasibility and the effectiveness of the design. The effects of the mean flow velocity inside the channel and the applied voltage across the channel on the performance of the separation are also discussed.