Abstract: The Helmholtz function and compressibility factor of associated mixture includes two contributions,physical interaction and chemical association.The molecular thermodynamic model for nonassociating systems developed in the authors previous work is used for the former.The Helmholtz function and compressibility factor due to self─association in mixtures with only self-association have been developed by considering additionally the association bonds between segments using shield-sticky model.The association paramters in model are calculated from the properties of pure associating fluid.Only one adjustable interaction parameter is used in the calculation of the physical interaction energy parameter of the mixture.Good agreements with experimental data have been obtained for calculating vapor-liquid equilibria of mixtures with only self─association under different pressures.