The mixing process of binary granular systems in a short rotating drum was investigated by performing discrete element simulations. The drum was asymmetrically operated with the left end wall being fixed and the right one rotated with the side wall. The modeled granular materials were made of particles with same size and density but different colors (red and yellow). These yellow and red particles were initially arranged side by side axially. We observed that the system inevitably went through a transient radial segregation state before reaching the final homogeneous mixing state. Depending on the filling degree and rotating speed, there existed totally three types of radial segregation pattern: yellow particles were enveloped radially by red particles (R-Y pattern), red partides were besieged radially by yellow particles (Y-R pattern), and the sandwich type R-Y-R pattern. Analyses of the axial flow field show that such transient radial segregating phenomeria were induced by different axial transporting characteristics of particles. The average axial motion of particles in the flowing layer and the location of the boundary between flowing layer and fixed bed layer influenced the transient radial configuration. (C) 2018 Elsevier B.V. All rights reserved.