Polystyrene (PS) microspheres with different sizes were prepared by suspension polymerization. The effect of initiator concentration on the average sizes and size distribution was evaluated. The results showed that the average size of PS microspheres increases with increasing initiator content, the optimal uniformity of PS microspheres was attained when the mass ratio of BPO to styrene was 1.5:100. The effect of stirring speed on the particle size was also investigated. The average size of PS microspheres reduced with the increase of the stirring speed.