The hardenability of steel is related to its chemical composition. About this relationship, multiple regression analysis of hardenability data was proposed to explain the effect of chemical composition on a hardenability of steel. To establish the formula for calculation hardenability, given hardenability curve(U.S.S. Atlas) were quantitatively analyzed by multiple regression analysis program of computer. The established hardenability model was applied to predict the hardenability of commercial steel fair well. The effect of chemical composition was also expressed quantitatively.