In travelling-wave electric fields, particles are propelled along electrode arrays by a propulsion force. The propulsion force depends on an applied frequency, voltage and size of particles. In this paper, we present the separation method of micro particles using the propulsion force and inclined gravity. The suspensions of polystyrene beads were used as the method to demonstrate the general application for the selective retention or transportation of particles. The efficiency of the method depends on the size of particles and mass density of particles. Additionally the method can measure the propulsion force on particles by adjusting the inclined angle.