Abstract: Correct prediction of the range of explosion hazardous areas during the loading of oil tank is significant for the safe operation of tankfarms. The mathematical and physical models of gas diffusion of oil tank are established, and numerical modeling is made for oil and gas diffusion process in the real scene during loading of oil tanks, considering the effects of oil properties, discharge rate, shape and position of the leakage hole and meteorological factors. When the ambient temperature is 25 ℃, a wind speed at 2 m above the ground is 3 m/s, atmospheric stability is D, environmental humidity is 75% and pump flowrate is 200 m³/h, the scopes of the hazardous areas of gasoline tank are defined as follows: for operation at the top of the tank, the areas within 0.823 m vertically to the top and 8.242 m to the ventilation valve center are considered hazardous. Through comparing the numerical simulation results with the calculation results by empirical formula, it is found that the former is closer to the measured value, which further verifies the accuracy of the method.