Most papers study loan guarantees based on the one-borrower and one-guarantee framework. In this study, firstly, we use option approach to construct a model, in which loan guarantees are analyzed under constant interest rate and multiple-borrowers and one-guarantee frameworks. We carry out simulations to investigate how the important parameters of borrowers and guarantee affect the values and default probability of guarantee contracts. Further, we also extend the above frameworks to the case of stochastic interest rates. We analyze how the stochastic interest rate components affect the values and default probability of guarantee contracts.