In this paper, the effect of geometric parameters of tube and die on the forming behavior of AA6061 step tube in hot metal gas forming process (HMGF) is investigated. For this purpose, empirical experiments and finite element simulations with ABAQUS software have been used. Investigations have been made at the different ratios of die to tube diameter (D/d) and the different ratios of tube thickness to diameter (t/d). A simple theoretical model for the relationship between these geometric parameters and the process parameters such as internal pressure and axial feeding is presented. The results show that under constant internal pressure and axial feeding conditions, the die filling percentage decreases with increasing the ratios of D/d and t/d. Also, in the constant D/d ratio, by increasing the t/d ratio to about 0.05, the die filling percentage reduces gradually, but with increasing t/d to 0.06, a sharp decrease occurs in the die filling percentage. Using different simulations, the internal pressure, and axial feeding are changed proportional to the t/d and D/d ratios. The results show that in accordance with the prediction of the theoretical model, the relative internal pressure and relative axial feeding should be increased linearly with increasing the t/d and expansion ratio , respectively, to give specimens with approximately the same die filling percentage.