We present a continuous size-dependent particle separator using a negative dielectrophoretic (DEP) virtual pillar array. Two major problems in the previous size-dependent particle separators include the particle clogging in the mechanical sieving structures and the fixed range of separable particle sizes. The present particle separator uses the virtual pillar array generated by negative DEP force instead of the mechanical pillar array, thus eliminating the clogging problems. It is also possible to adjust the size of separable particles since the size of virtual pillars is a function of a particle diameter, applied voltage, flow rate, etc. At an applied voltage of 500 kHz, 10 V(rms) (root mean sqaure voltage) sinusoidal wave and a flow rate of 0.40 microl min(-1), we separate 5.7 +/- 0.28 microm-, 8.0 +/- 0.80 microm-, 10.5 +/- 0.75 microm-, and 11.9 +/- 0.12 microm-diameter polystyrene (PS) beads with a separation purity of 95%, 92%, 50%, and 63%, respectively. The 10.5 microm- and 11.9 microm-diameter PS beads have relatively low separation purity of 50% and 63%. However, at an applied voltage of 8 V(rms), we separate 11.9 microm-diameter PS beads with a separation purity over 99%. At an applied voltage of 500 kHz, 10 V(rms) sinusoidal wave and a flow rate of 0.11 microl min(-1), we separate red blood cells (5.4 +/- 1.3 microm-diameter) and white blood cells (8.1 +/- 1.5 microm-diameter) with a separation purity over 99%. Therefore, the present particle separator achieves clog-free, size-dependent particle separation, which is capable of size tuning of separable particles.