According to the practices of banks in Taiwan, two interest rates will be applied to a commercial loan guaranteed by Small and Medium Enterprise Credit Guarantee Fund (SMEG), one for the guaranteed part, and the other the non-guaranteed part. Based on this rate discrepancies, this paper tries to evaluate the credit risk of SMEG's main guarantee programs by integrating the risk-neutral model with actuarial valuation principles, and to derive the optimal guaranty fees model. The empirical results show that the realistic subrogation payment is close to the total guaranty fees estimated by the proposed model. That is to say, this model has incorporated the accurate credit status of a loan, and to the best, this market information is also convenient to observe. It is hoped to make some contributions in controlling credit risk and establishing reasonable guaranty fee structures for SMEG.