Abstract: In the present study, a single particle growth model is developed to study the effects of initial catalyst size and initial catalyst active site concentration on the rate of polymerization and Molecular Weight Distribution (MWD) for styrene coordination polymerization over soluble metallocene catalyst. To describe the spatial-time evolution of monomer concentration profiles in growing particles, the Polymeric Multigrain Model (PMGM) is utilized. The PMGM is solved together with kinetic scheme to predict the rate of polymerization and MWD. The model simulation showed the presence of a significant distribution of monomer concentration across the radius of particles growth. It was further noticed that, the polymerization rate increase with increasing initial active metal concentration, while changing in the opposite direction under the influence of initial catalyst size. Additionally, the MWD increase with increasing initial active metal concentration and initial catalyst size.