The effects of particle size and/or density on particle interaction and overall behavior of granular flow were studied using the kinetic theory approach. The kinetic theory for granular flow was extended to mixtures of multitype particles assuming a non-Maxwellian velocity distribution and energy non-equipartition. Each type of particles was considered as a separate phase with different velocity and granular temperature. The resulting momentum equation for each particulate phase includes phase interaction arising from collisional pressure and particle-particle drag force. When applied to simple shear granular flow of a binary mixture, this model predicts well the energy non-equipartition and the stresses of particulate systems with diffc rent sizes and densities. (c) 2005 American Institute of Chemical Engineers.