Abstract: This article attempts to analyze the forming of the hat-section stiffener in the SPF/DB process for an integrally stiffened structure. Based on the fundamental equations of superplastic mechanics and the deformation mode in superplastic bulging of sheet metal into hat-section, the equations to describe the optimal pressure-time relationship in this process and the formula to compute the minimal thickness in theory are derived. Finally, a numerical example is given and discussed.