Abstract: Gives a formula for calculation of crude oil molecular weight and compares the results calculated from crude oil samples. It is concluded that the accuracy of crude oil component molecular weight affects the reliability of design calculation of crude oil processing, the molecular weight of C₆₊ can't be measured directly and various methods of calculation get different molecular weight; the bigger the molecular weight value applied, the higher the calculation value of lighter hydrocarbon production rate of C₆₊ component. The quality of oil product is changed with the effect of C₆₊ component molecular weight, the heavier the oil product, the more content of C₆₊ component, and greater effect and thus increases unreliability of design calculation.