Dispersion Polymerization of styrene with dodecyl mercaptan (DDM) used as a chain transfer agent was studied. Effect of concentration and addition of dodecyl mercaptan during polymerization have been investigated. As the DDM concentration was increased from 0~10wt%. the average particle diameter was increased. on the other hand decreased in polymerization rate and molecular weight. This study shows that the presence of DDM before and after the particle nucleation affect the particle size. molecular weight and rate of polymerization.