A ternary system solid-liquid equilibrium phase diagram is very useful to design industrial separation and purification operation. The ternary phase equilibrium for 3- methyl-4-nitrobenzoic acid (4,3-NMBA) and 3-methyl-2-nitrobenzoic acid (2,3-NMBA) in methanol and acetone at 298.15, 308.15, and 318.15 K was studied under atmospheric pressure in this work. In each ternary phase diagram, the saturated liquids divided the phase diagram into four regions, including three crystallization regions (pure 4,3-NMBA, 2,3-NMBA, and a mixture of 4,3-NMBA and 2,3-NMBA) and an unsaturated region. The crystallization region of 4,3-NMBA was larger than that of 2,3-NMBA in methanol and acetone at all evaluated temperatures. Moreover, the NRTL model was used to correlate the phase equilibrium data, and the interaction parameters of solute-solute were obtained. These parameters are of great importance to control the separation and purification process of 4,3-NMBA and 2,3-NMBA. To demonstrate the implementation of diagram application in industrial separation and purification, ternary equilibrium phase was analyzed and discussed.