Based upon a consideration of the similarity in the driving force of polymer crystallization to the mesophase formation from the melt state, the equilibrium melting point of chain polymers relating to the details of molecular structure and the impure environment of crystallites was studied by classical lattice statistics and dynamic Monte Carlo simulations. The compact packing energy change of local chains during crystallization can be represented as an orientation-dependent attraction in the lattice model. The results as a function of driving force, chain length, content of random comonomers, and content of the diluent like small molecules or noncrystalline chains were compared with the semiempirical expressions that have been verified for several polymers. Good agreements were obtained. As a result, the classical lattice statistics can predict a disorder-order phase transition as well as a phase separation behavior in a mixture system containing polymers, and supply the thermodynamic background to explore details of these behaviors in computer simulations.