Through the equation of balance sheet between assessment and debt, this paper reformulates the function with two stochastic process of credit risk and interest rate risk under the full consideration of the profile of the losses of bad debts of the commercial banks, deduces the function of loan rate under the maximum utility of profit margin of commercial banks. The Beyer's estimation is used to analyse the changes of the loan rate when the extent of the risk averse of commercial banks. The risk averse extent is raised from the Arrow-Pratt risk averse to Ross risk averse. It is pointed out that the more risk averse of a commercial bank, the even higher the interest rate the bank charges to its borrower.