In semi-batch polymerization reactors, due to changing monomer concentration, the reaction rate varies non-linearly with the time. The heat transfer characteristics change by the large changes in the viscosity of the polymer solution over the reaction and by the environmental conditions like temperature. These factors complicate the automatic control of polymerization processes. In this study, considering operational constraints, optimum reactor temperature for the desired conversion and molecular weight in minimum time was tried to track using self-tuning PID controller. Tuning parameters of controller were obtained by using Genetic Algorithm. The fitness function for GA was chosen to be the integral of the absolute value of the error (IAE). The results show that STPID controller with genetic algorithm controls the temperature of the semi-batch polymerization reactor very well.