We propose an equation of state for fluid mixtures of water+ammonia system which is expressed in Helmhotz free energy as a function of temperature, molar volume and mole fraction. The IAPWS Formulation 1995 for water and equation of state by Tillner-Roth et al. for ammonia should be used for each pure component with our equation. We applied the principle of corresponding states with two-fluid model to our equation for fluid mixtures. Accuracy of our equation was evaluated by means of comparison with the experimental data for thermodynamic properties. The comparisons show the accuracy of our equation : ±0.03 in mole fraction of VLE in liquid phase, ±0.04 in mole fraction of VLE in vapor phase, ±2% in saturated liquid density and ±2% in PVTX properties