The effect of the grain size distribution on the micro structure of granular materials has not been fully understood. Especially, few works is currently available on the coordination number of the well-graded granular mixtures due to the difficulty and uncertainness of experimental measurement. This research aims at studying systematically the effects of grain size distribution on the coordination number and constructing a model to predict the frequency distribution of the coordination number of granular materials on the basis of their grain size distribution. Using a two-dimensional Discrete Element Method, nine types of disk assemblies were packed densely or loosely and their coordination numbers and void ratios were measured. Simulation results indicated that the coordination number and the void ratio generally decreased with increasing grain size dispersion. The authors found a general relationship between the number of contact points and the particle size which can be applied to granular materials having various grain size distributions. In the case of densely packed assemblies, this relationship is applicable to predict the frequency distribution of coordination number only from their grain size distribution. (C) 2011 Elsevier B.V. All rights reserved.