Abstract: The major cause which exists a troublesome problem in unloading the road tanker in winter is the long travel and fast heat transfer of tank, which makes the temperature of crude oil loaded decreased below the condensation point. The equations that the oil temperature whthin the tanker is changed with time are derived by means of establishing the model for non-stable heat transfer and analyzing the process of heat transfer. Taking the tank of Steyr road tanker as an example, the corresponding values of temperature versus time and the curve of temperature versus time are obtained. By comparing the calculated values with measured values, the maximum relative error is only 3.5%, which shows that the equations and the calculation method are practical and suitable for the design.