The instability and the localization process of unstable plastic flow during constant pressure bulging of superplastic Ti-6Al-4V sheets are analyzed. A generalized stability criterion is developed by generalizing Hart's stability criterion[1] in terms of strain and strain rate. According to the stability criterion, a new concept of the "flow localization factor (FLF)," which enables quantitative description of the localization process of unstable plastic flow, is presented. A finite element model that simulates experiments on constant pressure bulging is applied to compute the FLF at the fracture point. it is found that the localization process during constant pressure bulging can be divided into three stages: (1) the developing period of initial localized flow, (2) the steady stage of strain concentration, and (3) the accelerating stage of strain concentration, which results in the final fracture.