In this study, we investigate about the effects of molecular diffusion in the pore network in the high temperature- high pressure condition at the isothermal condition. In this study, we construct a network which consists of pores and throats that connects together irregularly. The gas has been injected from the fracture and starting to mass transfer with liquids which are in the throats. Because the placement of gas fluid near to the liquid fluid, molecules of gas phase goes to the oil fluid and molecules of oil fluid goes to the gas phase. This mechanism causes that the oil which exists in the throats has gradually evaporated and cause of saturation of throats by gas fluid. Transfer mechanisms in the pore network has been done by two main mechanisms: capillary forces and molecular diffusion. In this study, molecular diffusion in the gas phase, flow caused by capillary forces and evaporation at the interface of gas-liquid has been modeled. In previous projects, the effect of gas diffusion in the oil phase was ignored and the laws of ideal gases were applied to the thermodynamic calculations of the gas fluid. As a result, the calculations related to molecular diffusion and evaporation at the gas-liquid interface did not match the actual fluid conditions in the reservoir. In this project, diffusion of gas in the oil phase is also considered, and for thermodynamic calculations, the equilibrium equations of gas-liquid at the interface has been used. On the other hand, there are calculations on a network extracted from a real rock to bring the results closer to reality. We concluded that with increasing temperature, the time of evaporation of pore network is decreased. Also variation of liquid moles, rate of decreasing density of liquid and number of liquid clusters, is increased. In this study we investigate the effect of fluid pressure on the evaporation process and we observe that with increasing the pressure, the time of evaporation rate, is increased. Also we find that with increasing fluid pressure, variation of liquid moles, rate of decreasing liquid density and number of liquid clusters, is decreased. In this study, also we concluded that, with lightening gas fluid and liquid fluid, have been affected on the time of evaporation in the pore network and cause of increasing the rate of evaporation at the gas-liquid interface which this decreases the time of evaporation. Also we concluded that with lightening the oil and gas fluid, variation of moles of liquid, rate of decreasing oil density and number of liquid clusters, is increased.