The concept of the volume fraction at chain end is proposed, which is the conditional probability for a site having been occupied, knowing that an adjacent site is occupied by polymer end. The overall entropy of polymer/solvent system is separated into two fundamentally different parts, i.e., the translational entropy and the conformational entropy. Based on these a theory of polymer solutions is established. When a mean-field approximation is introduced, Flory-Huggins (FH) theory is recovered. The FH interaction parameters and spinodal curves of the polystyrene/cyclohexane system are calculated and compared with the experimental data. The good prediction of them two is achieved. (C) 2004 American Institute of Physics.