The mixing proceises of equal-sized acrylonitrile butadiene Styrene copolymers beads in a rotating drum were simulated using the Discrete Element Method (DEM) at different rotational speeds and filling degrees. The contacting parameters of the DEM models were determined by a series of experiments including high rebound test, friction and wear test, stacking angle test and L-box test. The validity of the DEM simulations was confirmed by comparing the numerical results with the experimental ones. Quantitative criteria of particles mixing performance which is characterized by the mixing degree and mixing time were proposed based on the mathematical statistical analysis of simulation results. A new parameter C which is defined as the ratio of the active region area to the whole bed area was suggested to describe the active region in the mixing process. Our numerical results reveal that the parameter C has a significant influence on the mixing time but little influence on the mixing degree. With the increase of the rotational speed and the decrease of the filling degree, the parameter C increases but the mixing time decreases. (C) 2017 Elsevier B.V. All rights reserved.