A model is proposed to describe the collision rate of small particles suspended in a turbulent system. The model combines the possible collision mechanisms : 1) collisions due to the relative velocity between fluid and particles, and 2) collisions due to the turbulent diffusion of particles. This model also accounts for the effect of particle concentration on the collision rate. It was found that the turbulent diffusion of particles plays an important role in the collision of equally sized particles as well as of unequally sized particles. The model predictions also show that the collision rate of particles is strongly affected by the concentration of solid particles and by the turbulence intensity. Much more reliable predictions than previously possible have been obtained with the present model.