Based on the coupled mode theory, the bistability performance of nonlinear Bragg gratings is analyzed theoretically in terms of elliptic integration. The analytical expression describing the relation between the input intensity and the output intensity is presented. Consequently, the dependence of the bistability on the gratings inner parameters is investigated numerically. The results show that, the bistability performance of the nonlinear Bragg gratings depends not only on the product of the couple coefficient and the grating length but also on the couple coefficient and the gratings length respectively.