A two step polymerization process for manufactureing expandable polystyrene (EPS) has been considered in this study. The first step involves uspension polymerization of styrene in a stirred batch reactor. In the second step, n-pentane is loaded into the reactor as a blowing agent. A semi empirical model describing the kinetics of suspension polymerization of EPS has been presented. The kinetics has been modeled on the basis of the free volume theory accounting for the change in termination, propagation, transfer to monomer and initiation rate constants as polymerization proceeds. A dual initiator system has been used in this process which one of them has higher half-life than the other. Addition of pentane to the system leads to a strong decrease of polymerization rate and Delayed the occurrence of gel effect due to increase in total free volumeof the system.The parameters of model adjusted with an industrial stirred batch reactor located in Tabriz Petrochemical Company (TPC). Thermal model has been developed according to energy balance on reactor and jacket.The final model has been presented with simultaneously solving kinetics and thermal equations and results of model have been compared with expremental data from TPC reactor.and show execlent agreement with these data.