In this work, we experimentally and numerically investigated the mixing/segregation behavior of particles in an unbaffled rotary drum. We analyzed different combinations of binary mixtures that varied in density and size. We also quantified the effect of filling degree and rotational speed of the drum on the mixing index of each binary mixture. We used the discrete element method (DEM) for the simulation study, in which we determined the input parameters by both experimental measurements and calibration. The results reveal that the simulations using experimentally measured DEM input parameters had the best agreement with the experimental responses (repose angle and radial mixture). The experimental and simulation results enabled us to analyze the significant effects of filling degree and rotational speed on the granular radial and axial segregation of different binary mixtures. (C) 2020 Elsevier B.V. All rights reserved.