Structural system reliability based on ultimate strength analysis is more favorable in assessing the safety of ship structures due to the high degree of indeterminacy and the complicated marine environment. In the past, most of the structural system reliability studies were performed for framed structures with simple failure modes. However, such studies are not applicable in ship structures since the failure modes are much more complicated in present investigation. The typical stiffened box structures are examined in this study to do the system reliability analysis. The main objective of this project is to evaluate structures with the system reliability analysis. Based on the ultimate strength analysis and the advanced first order second moment (AFOSM) reliability method, the system reliability index or the probability of failure of a structure can be obtained. In this research, the β- unzipping method, the Ditlevsen’s bounds method and the finite element analysis (MSC/NASTRAN) are used to find the bounds of the probability of failure of structures.