A model estimating local particle dispersion/aggregation behaviors in a suspension has been developed for a non-uniform shear flow. In this model, the size distributions of particle clusters were calculated with taking a balance of Brownian and shear coagulations and shear breakup. A numerical simulation was performed for a two-dimensional backward-step flow as a non-uniform shear flow. A flow experiment using a micro-channel with a rib was also conducted for the verification of the present numerical analysis. The solid volume fraction of the suspension was set at 0.01 and the step height Reynolds number was kept constant at 1.5×10^-4 both for numerical and for experimental studies. The numerical results obtained in this study were rather reasonable compared with the experimental data. From this, it was found that the present numerical model is promising for estimating particle dispersion/aggregation in a non-uniform shear flow.