In the metal forming processes, the process variables (such as forming speed) have significant effects on the forming parameters. In this study, using the motion equations (instead of the equilibrium equations) in the biaxial stretching analysis of the Sheet, the evolution of initial in homogeneity has been simulated. Adopting work hardening and strain rate hardening characteristic for material, the effects of forming speed on the shape and position of the forming limit diagram (FLD) are investigated theoretically over a wide range of strain rate. Furthermore, the combined effects of the strain rate and material intrinsic parameters (density, strain hardening component and strain rate sensitivity index) on the critical strain are obtained in the case of plane strain loading. The analysis indicates that after a certain forming rate, by increasing the strain rate, localized necking is retarded and formability of Sheet improves monotonically because of inertia effects.