In petroleum reservoirs, estimation of the rate of mass transfer caused by molecular diffusion is necessary for determining the amount of gas diffusing into oil in gas injection projects. The key parameter for calculating the mass transfer rate is diffusion coefficient of gas into oil. Hence, in enhanced oil recovery projects, the knowledge of this key parameter for designing the injection projects is essential. Oil swelling is a common phenomenon that is observed in gas injection projects and should be taken into consideration for modeling of diffusion process in enhanced oil recovery projects. Mathematical modeling is a good tool, which may help in predicting this parameter. In this study, an attempt has been made to develop a mathematical model for predicting the gas diffusion coefficient for the system in which the gas is injected into oil through a water-blocking film. The model equation has been solved with appropriate initial and boundary conditions to obtain a series of type curves similar to those that are used in well testing. The results are classified in two groups, oil and water phases, and are presented in dimensionless form. The effect of swelling varies with the variation of the Peclet number. It may be concluded that, by increasing the Peclet number, the role of convection mechanism is more prominent.