In this paper, aggregation/dispersion behaviors of particle clusters in a non-uniform shear flow have been numerically analyzed using a thixotropy model for estimating local cluster size distributions. The effects of the viscosity of the dispersion media, the solid volume fraction and the Reynolds number on the cluster size distribution in a two-dimensional flow between parallel plates were investigated in order to find the primary factor determining the cluster size distributions. The viscosity of the dispersed media, the solid fraction and the Reynolds number were changed from 195 to 1950 Pa.s, from 0.10 to 0.25 and from 0.1 to 1.0, respectively. From the results, it is found for each case that large clusters are observed at the central area of the channel, but it becomes smaller near the wall where high shear is added. The cluster size near the wall is dominated by the shear stress. On the other hand, it is found that the cross-sectional averaged cluster size depends on the bulk velocity under the present conditions. As the cluster size near the wall can be correlated rather well with the bulk velocity, it is concluded that the bulk velocity is a convenient and useful factor for the dispersion/aggregation behaviors of fine particles under the present conditions.