Superplastic blow forming processes are simulated with a finite element method which adopts the convective coordinate system. The finite element code developed calculates the deformed shape, the thickness distribution, and the strain rates as well as the optimum pressure cycle. The code is applied to the square cup blow forming process to predict the optimum forming pressure cycle. The numerical results also provide the pole height, the intermediate deformed shapes and the thickness distribution with respect to time. The validity of the code developed is demonstrated by comparing the numerical result with the experimental result.