The mixing behavior of particles in a rotating drum mixer (RDM) was analyzed by experiment and Distinct Element Method (DEM) simulation in order to discuss the effect of particle density. In both of the experiment and the DEM simulation, mixing behavior of alumina (3600 kg/m(3)) and stainless steel (7930 kg/m(3)) particles in the rotating drum was observed and mixing degree was evaluated. In the experiment, most of the alumina particles with lower density exist close to the container wall and in the vicinity of the surface of particle bed. The mixing behavior of DEM simulation is comparatively similar to that of the experiment, and it is confirmed that the DEM simulation has high reliability for simulating the mixing behavior. The DEM simulation was used to analyze the influence of particle density on mixing behavior and to make clear the segregation mechanism in a binary system. When the particle density ratio is larger, the mixing behavior in a rotating drum is strongly affected by the ratio. In order to analyze the mechanism of the segregation quantitatively, the new index related with the mobility of particles was developed. This result of the index indicated that the particles with lower density could move easily in comparison with higher density particles, and the difference between the mobility of those materials cause the segregation. The rotating drum with lifters (RDM_L) was adopted to improve the mixing state of particles, and particles behavior in the drum was simulated by the DEM. The mixing state of particles becomes comparatively uniform and the particle segregation would not be observed. These results indicate that lifters could control the behavior of higher and lower density particles and enhance particles mixing. (C) 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.