With cell model, the orientional order parameter, the positional order parameter and the orien-tion and position coupling order parameter are defined on cell center. By choice of interaction potential of biaxial molecules with a permanent electric dipole moment transverse to the molecular long axis, the free energy of the system is obtained . A differential equation of the director is derived by calculating the variation with respect to free energy, and the director tilt and helical change about the normal of molecular layer are obtained. The approximate formulas of tilt angle and pitch for three classic SmC* Liquid crystals of DOBAMBC, etc., are presented. The calculated results agree satis-factorily with the experimental ones.