We describe the lateral displacement of a particle passing over a planar interdigitated electrode array at an angle as a function of the particle size. The lateral displacement was also measured as a function of the angle between the electrode and the direction of flow. A simplified line charge model was used for numerically estimating the lateral displacement of fluorescent polystyrene (PS) beads with three different diameters. Using the lateral displacement as a function of particle size, we developed a lateral dielectrophoretic (DEP) microseparator, which enables continuous discrimination of particles by size. The microchannel was divided into three regions, each with an electrode array placed at a different angle with respect to the direction of flow. The experiment using an admixture of 3-, 5-, and 10-microm PS beads showed that the lateral DEP microseparator could continuously separate out 99.86% of the 3-microm beads, 98.82% of the 5-microm beads, and 99.69% of the 10-microm beads, simply by using a 200-kHz 12-Vp-p AC voltage to create the lateral DEP force. The lateral DEP microseparator is thus a practical device for simultaneously separating particles according to size from a heterogeneous admixture.