This paper presents the optimum exponent of Barlat-Lian (1989) yield function which defines the yield surface shape for different materials under plane stress condition. The predictions of the optimum exponents of Barlat-Lian (1989) yield function are derived and then compared with the experimental data for these materials to allow a better representation of the plastic behavior of the orthotropic sheet metals. The results show that contrary to what it was supposed to be up to now the optimum exponent of Barlat-Lian could be have different values for verity of face and body centered cubic types of materials case by case.