This paper presents a new sorting method for acrylonitrile-butadiene-stryrene (ABS) and poly styrene (PS) by means of microwave cavity resonator perturbation. In this method, a small piece of sample material is inserted into a cavity resonator at a point when the electric and magnetic fields mark the maximum and the minimum values, respectively. Experiments were carried out with use of 1-, 2-, 5-, and 10-GHz resonators and ABS and PS samples of various sizes. Loss angles of polarizability (tan χ) exhibited differences large enough to achieve accurate sorting between ABS and PS.