Air-table is one of the specific gravity separators which have been expected as an effective dry separator for waste products such as plastic or glass. The objective of this study is an application of DEM to describe the basic behavior of plastic particles in an air-table separation. The mechanism of particles separation on the table has not been well-known because air-table contains many instrument parameters such as slope and shape of the table, height of the riffle, angle, amplitude and frequency of vibration and wind velocity. In this study, two types of particles which have same diameter and different density were simulated and 3 dimensional position and velocity of them were investigated in the various conditions of instrumental parameters. DEM simulation results showed that the riffle on the table play an important role for the particles separation.