Abstract: Phase equilibrium relations of H₃BO₃-NaNO₃-H₂O ternary system at 25 ℃ were studied by wet-residue method. Meanwhile，main physico-chemical properties，such as density，refractive index，conductivity，and pH of the equilibrious liquid were detected.Results showed that the ternary system belonged to a simple water-salt ternary system.Mass fractions of H₃BO₃ and NaNO₃ in the total saturated solution were 2.75% and 32.4% respectively，and there was no formation of complex salt and hydrate.Density and refractive index of the equilibrious liquid were calculated by empirical equations.Calculated values were in agreement with the experiment values on the whole.

Key words: sodium nitrate, boric acid, ternary system, phase equilibrium