A kinetic model including two types of active sites was developed for the polymerization of ethylene in slurry with a polymer supported Ziegler-Natta catalyst to predict monomer conversion, polymerization rate and molecular weights. The model includes activation of active sites, propagation, deactivation, chain transfer by hydrogen and the effect of monomer diffusion through the polymer layer around the catalyst particles to the active sites. Transformation of active sites from type I to type II is allowed. The polymerization data were obtained from a semibatch slurry reactor under constant temperature and pressure. The experimental program was carried out according to a central composite design with monomer partial pressure, catalyst loading, hydrogen to monomer ratio, temperature, and catalyst particle size as process variables.