This paper quantitatively and qualitatively evaluates the mixture and segregation processes in an unbaffled rotary drum induced by particle size and density differences. We also assessed the initial particle loading effect using side-side and top-bottom particle configurations. The suitable segregation index and the segregation rate quantified the mixture dynamics. We carried out numerical simulations based on the three-fluid Eulerian model and compared them with experimental data obtained under the same conditions. The particle initial loading, the particle physical properties and the drum operating condition strongly affected the particle segregation in the rotary drum. The simulation predictions showed similar results when compared with the experimental observations. (C) 2016 Elsevier B.V. All rights reserved.