An improved multigrain model designed to simulate the polymeric particle growth and morphology in the gas phase polymerization of butadiene was developed. In the model, the effects of intraparticle heat and mass transfers, heat and mass transfer resistances at the particle boundary layer, sorption of 1,4-butadiene in 1,4-cis-polybutadiene, and intrinsic kinetics on the polymeric particle growth and morphology were considered. An improved numerical method was also proposed.