High pressure is involved during injection molding operation, especially in the packing phase. Cracks in the mold often occur as a result of high cavity pressure. In this study, the structural analysis of an injection mold was performed to investigate the stress concentration in injection mold. The structural analysis included stress distributions and stress concentration according to cavity pressures and mold designs. The causes of cracks were investigated through the computational results for stress concentration locations and the magnitude of stress. Two commercial programs were used and various boundary conditions were applied to obtain reliable results. This study found that the locations of high stress concentrations were coincident with the locations of cracks and that the weak mold design was responsible for these cracks. Mold designs such as interlocking designs exerted a strong influence on the robustness of the mold. It was verified that the analysis method used in this study can be applied to evaluate the robustness of injection molds.