The mechanism of a novel dielectrophoretic particle trapping system that selectively captures specific-size particles from a mixture of various-size particles is investigated. The results show that particles that fall within a certain range of vertical positions and velocities are captured, which enables size-selective trapping. The highest trapping efficiency is achieved at unexpectedly large microchannel heights, in contrast to conventional approaches that use shallow channels. This enables the rapid isolation and concentration of specific-size particles in a single step, which overcomes significant inefficiencies of current separation techniques caused by multiple treatments and dilution.