In this paper, we present a novel, simple lab-on-a-chip device for continuous separation of particles by size. The device is composed of a straight rectangular microchannel connecting two inlet reservoirs and two exit reservoirs. Two asymmetric, 3-D electrodes are embedded along the channel wall to generate a non-uniform electrical field for dielectrophoresis. Particles with different sizes are collected at the different exit reservoirs. Main flow is induced by pressure difference between the inlet and the exit reservoirs. The device is used successfully for the separation of the 5 and 70 Em latex particles and for the separation of yeast cells and white blood cells. A numerical simulation based on Lagrangian tracking method is used to simulate the particle motion and the results showed a good agreement with the experimental data.