With cell model the integral equation of distribution function is obtained by minimiza-tion of the free energy function. The orienrational order parameter, the positional order pa-rameter and their coupling are defined. By a proper choice of the interaction potential, the approximate solution of the distribution function is obtained, and also for free energy. We find that SC-SA phase tdansition can be either the second order or the first. We have calcu-lated the temperature dependence of the tilt angle and of the order parameter, as well as the ent-ropy change and the heat capacity near the critical point for TBBA, TB8A and NOBA. The variation of the phase transition temperature with molecular chain length for homologous series of TBBA and 4-n-alkoxybenzylidene-4'-aminozobenzenes are also calculated. Our calculations agree satisfactorily with experimental results.