Abstract: A molecular thermodynamic model for Ising lattice was proposed based on the statistical mechanics and on the results from Monte Carlo simulation, which correctly takes the effects of temperature and composition on internal energy into account. The model gives a very satisfactory prediction of liquid-liquid co-existing curve of Ising lattice. Comparisons with the MC results show that the new model is much better than that of Flory-Huggins theory, and also better than Freed theory.