Abstract: A thermodynamic model of vapor-liquid equilibria in NH3-H3PO4-HO system has been studied. In the vapor liquid equilibria calculation by using extended Pitzer equation, water activity and solute activity coefficients in aqueous phase were obtained. By the modified Redlich-Kwong equation of state, vapor phase fugacity coefficients were obtained. Based on the assumption of Edwards et al (1975) of the molecular thermodynamics of aqueous weak electrolytes, a molecular thermodynamic correlation was established to calculate vapor liquid equilibria in NH3-H3PO4-H2O system. The correlation model calculated data agreed well with the experimental equilibrium vapor pressure data up to concentrations 25 mol/kgH2O.