One issue to consider while designing and constructing steel girders reinforced with carbon fiber-reinforced polymer (CFRP) plates in bridges is debonding failure. Previous studies showed that the parameters such as characteristics of material, load, adhesive, and CFRP plates have an effect on the failure probability of steel girder, which is represented by the reliability index. Therefore, this study analyzes the reliability indices of steel girders in bridges strengthened with CFRP plates to clarify the effects of debonding failure. Debonding and strength limit states are used to compare differences in reliability indices of different design scenarios. Strength and debonding margin the functions for the strength limit state and debonding limit state will be established in this study. The probability of failure is determined by a Monte Carlo simulation (MCS). It is found that the reliability index of debonding limit state is much lower than that of the strength limit state. This shows that the debonding failure should be considered significant in the reliability analysis of steel bridge girders strengthened with CFRP plate.