In this paper, liquid-liquid equilibrium (LLE) data of the quaternary system nitric acid + calcium nitrate + water + tri-n-octylamine (TOA) was investigated at 298.15 K and under atmospheric pressure for different mass percentages of nitric acid and calcium nitrate in the initial aqueous phase. The densities of aqueous phase (rho(a)) and organic phase (rho(o)) were measured. The distribution coefficients of nitric acid (D-1) and water (D-3) and the separation factor of nitric acid against calcium nitrate (beta(1)(2)) were calculated. Based on these experimental and calculated data, the selectivity and capability of TOA for extracting nitric acid were evaluated. The reliability of LLE data was tested by Hand and Othmer-Tobias equations. Both of the equations showed good linear correlations. The electrolyte non-random two-liquid (eNRTL) model was used to correlate the experimental data, and the binary interaction parameters and pair parameters were determined. The root-mean-square error value (1.29%) was calculated for the eNRTL model, indicating that the excellent results for this model were suitable to determine the LLE data of this quaternary system.