Abstract: By introducing effective chain - insert probability into the excess entropy and adopting Wohl expression for the excess internal energy, a molecular thermodynamic model for a multicomponent system based on the Flory - Huggins lattice model has been established. For binary system, when r1equals to 1 and r2 varies from 1 to 10000, the results calculated with the new model is more consistent with computer - simulation data than those calculated with Freed theory. Under other conditions, both theories are consistent . Being a mean - field theory, Flory - Huggins theory can not give good results. For a ternary system, the coexisting regions calculated with the new model and Freed theory are very close to each other, but those calculated with Flory -Huggins theory are different. Besides, the new model can describe phase behavior with loop spinodals or binodals .