A two-phase mass transfer model has been developed to predict the polymer molecular weight for the semibatch melt polymerization process of polycarbonate, in which the reaction melt viscosity is high and the mass transfer of volatile by-product is the rate-controlling step. The model simulation results have been compared with the experimental data reported in the published literature regarding the number average molecular weight and the reaction product composition. The mass transfer coefficient for phenol [(k_La)_P] in the melt polymerization process of polycarbonate has been estimated.