Maximum thickness reduction ratio is used to predict sheet metal forming limit in the numerical simulation of forming process. The maximum thickness reduction ratio under different stain path is not a constant for the same material. The effect of strain path and strain hardening exponent on forming limit is considered. The relationship between the maximum thickness reduction ratio that the material can obtained and the strain path between tensile to equi-biaxial is established. The parameter in the criterion can be determined by tensile experiment combined with numerical simulation of the same forming process. Then the limit strains under other linear strain paths between tensile to equi-biaxial can be determined by the criterion combined with numerical simulation of corresponding forming process. Forming limits of three kinds of sheet metals are predicted with the modified maximum thickness reduction ratio criterion. Good agreement is achieved between the predicted data and the experimental data.