In this work, the activity coefficients of sodium nitrate in the ternary system NaNO3-Cu(NO3)(2)-H2O were experimentally determined at 298.15 K in the range of total ionic strength I = 0.01 - 1.00 mol.kg(-1) at different ionic strength fractions of Cu(NO3)(2) with y(b) = 0.20, 0.40, 0.60, and 0.80 from the cell potential method of the cell without liquid junction: Na-ISE vertical bar NaNO3 (m(1)), Cu(NO3)(2) (m(2))vertical bar NO3-ISE. The Na-ISE and NO3-ISE used in this work have good Nernst response, which indicates that the cell potential method can be used to measure the abovementioned systems with high accuracy. The Nernst equation is used to directly calculate the mean activity coefficients of NaNO3 in the mixed solution. The relationship diagram of the mean activity coefficients of NaNO3 in the ternary system NaNO3-Cu(NO3)(2)-H2O is drawn. The experimental data are analyzed with the Pitzer model to fit the corresponding mixed ion interaction parameters theta(Na,Cu) and psi(Na,Cu,NO3), which required Pitzer equations. The osmotic coefficients, mean activity coefficients of Cu(NO3)(2), water activity, and the excess Gibbs free energy of the mixture solution were also calculated.