A wrinkling is the instability phenomenon influenced by material properties, shape geometry, forming conditions, stress state, etc. The wrinkling is considered as a critical defect in appearance of product. Many wrinkling prediction methods using thickness strain distribution and farming analysis have been proposed. The wrinkling, however, is not easily predicted precisely by these methods. In this study, the region in the biaxial plane stress state is modeled with a rectangular plate introducing the effective dimension, and critical stress values for the wrinkling are calculated. Prediction index for the wrinkling is then evaluated by normalizing the actual stress with respect to the critical stress. In order to show the validity and efficiency of the method proposed, the wrinkling prediction for a squared sheet in the uniaxial tensile stress and auto-body front finder panel is performed.