In this investigation, the mutual solubilities for the ternary o-nitrobenzoic acid + m-nitrobenzoic acid + ethanol system were measured at (273.15, 283.15, and 293.15) K, respectively. The isothermal phase diagrams of the system were constructed on the basis of the measured solubilities. Two solid phases were formed and confirmed by the Schreinemakers wet residue method, and the two were identified as o-nitrobenzoic acid and m-nitrobenzoic acid. The solubility fields of two solid phases were observed front the phase diagram. The phase diagrams of the ternary system are similar at the different temperatures. The crystallization regions of 2-nitrobenzoic acid and 3-nitrobenzoic acid increase as the temperature decreases. The solubility data and the ternary phase diagram can provide the fundamental basis for separation of the mixture of isomeric nitrobenzoic acids.