Abstract: A series of experiments were carried out to explore the characteristics and mixing mechanism of oil shale grains groups with diameter of 0～12mm in rotary kiln. Influences of rotation speed，vessel filling percentage and lifting flight forms on mixing efficiency were analyzed. Results showed that the particle group movement traces and mixing process was significantly affacted by the vessel filling percentage，the rotation speed and the lifting flight forms. With the increase of filling rate，the particle size was easily to produce separate effects；however，with the decrease in speed，the mixing of particle size with rectangular lifting flight was better than that of straight lifting flight.