From more than one uniaxial tension test curve, it can be found that mechanics property of sheet metal is inhomogeneous. Based on this truth, a new method for prediction of forming limit of sheet metal is proposed. Using this method, theoretical FLDs are obtained under proportional loading path based on Hill48 yield criterion. The effects of several material parameters on forming limit are obtained. Similarities and differences between this method and M-K instability theory are analyzed and some advices on the application of this method are given.