Material constitutive modeling is an important aspect in the continuous improvement process for sheet metal forming simulation and analysis. In this study, a sensitivity study of material constitutive parameters on forming simulation results is carried out for two different sheet metal parts. Three different yield criteria are evaluated in the simulation including isotropic yield criterion, Hill's 1948 anisotropic yield criterion and Barlat's non-quadratic anisotropic yield criterion. It is found that the forming results depend upon the yield criterion used in the simulation. Both thinning and stress in the formed part are very sensitive to the change in anisotropy value (r-value). Effects of strain rate sensitivity of a material on forming results are demostrated through the use of two different stress and strain data from different tensile speeds. It is also found that forming results such as thinning are very sensitive to the strain hardening behavior of the material.