This paper presents a numerical study of the flow and segregation of particles over flat blades in a vertical cylindrical mixer by using discrete element method. Simulations are conducted on binary mixtures of spheres with different size ratios. Similar flow features are found for all the cases considered: particles rise forming a heap in front of the blade. One part of this vertical flow goes over the blade and toward the bed surface; another part goes down to the base of the heap in front of the blade to rejoin the flow toward the blade. In addition, there is a recirculating zone in front of the blade. Changing blade speed affects the mixing kinetics but not the mixing quality. Large particles are mainly inthe top layer of the mixing bed, while small particles remain in the bottom of the mixing bed and in front of the blades. Analysis of the force acting on the particles shows that at the initial stage, large particles always receive a relatively large vertical force than small particles leading to segregation that can be maintained at a later stage. Different mixing behavior is quantified for mixtures of different size ratios.