The dominant mechanism affecting the flow behavior of granular materials is the random motions of particles that results from the interactive collisions between particles. The velocity fluctuations induce the segregation in granular flows. The different densities of granular materials with different filling layers and packing ratios were used to investigate the segregation mechanisms in a vibrated granular bed in this study. The image processing techniques were employed to track the motion of granular materials. The fluctuating velocities of granular materials were measured under segregation in a vibrated granular bed. The segregation phenomena depend on the packing ratio and the filling layer of the stainless steel-glass mixtures. At the fewer filling layers, the heavier stainless steel beads migrate to the bottom regions with the lower granular temperature, so the granular temperature plays a significant role on density segregation. For the higher filling layers, the bulk convection motion is the main mechanism affecting segregation, so the heavier beads clump to the convection center and the clump size enlarges with the increasing packing ratio of heavier beads due to the bulk convection motion. (c) 2010 Elsevier B.V. All rights reserved.