Finite element simulations were conducted to investigate the influence of grain growth in the superplastic blow forming process. A microstructure-based constitutive model considering grain growth effects is proposed and used in the simulations. Also, a grain growth rate equation accounting for both static and dynamic grain growth is implemented. The simulations were made using a 2D plane-strain model for constrained blow forming and an axisymmetric model for free bulging. These two models showed different features during the forming stages. However, the forming pressure-time curve and the thickness distribution obtained by both simulations explained well the deformation hardening induced by the grain growth during superplastic forming. This study shows that grain growth is an important factor in determining the material behavior during superplastic deformation.