Abstract: In order to save energy and metallic material,it is very important to calculate the logarithmic mean temperature difference between the two kinds of fluids and the superficial area of the heat exchanger accurately.The traditional calculational method is too complicated,but the error of simplified method is too significant,so we deduce the formulas for logarithmic mean temperature difference between the cold and hot fluids in a heat exchanger in consideration of their thermophysical properties and the formula for sueperficial area of the exchanger.The formulas are easy to use and give accurate results.