Abstract: An equation of state for square well chain fluids with variable range (SWCF VR) was applied to calculate thermodynamic properties of amino acid aqueous solutions. The model parameters for 17 amino acids were obtained by combining the densities of amino acid aqueous solutions. SWCF VR can give satisfactory prediction by only using molecular parameters of amino acid and water, and the overall average relative deviation (ARD) of density was 0.43%. Using an adjustable parameter associated with temperature, the density of binary solutions of amino acids can be correlated with a high precision, and ARD was only 0.012%. Coupling with an adjustable parameter from density data, SWCF VR can successfully predict the solubility of amino acid selected in water.