In this paper, b values of some seismic zones were calculated by means of modified magnitude-frequency relationship logN=b₁+b₂M+b₃M². The results show that modified magnitude-frequency relationship improved likelihood precision remarkablely. The residual of modified magnitude-frequency relationship was reduced 5.5 times maximally as against traditional G-R relationship. Upper magnitude of seismic zone was obtained conveniently. If there are enough time length data, obtained upper magnitude will rather satisfactory.