Phase diagram of a tertiary mixture composed of two solid components (2,4-DNT and 2,6-DNT) and one liquid component(ethanol or sulphuric acid) was first set up by solubility approach. The solubility of both 2,4-DNT and 2,6-DNT in ethanol or sulphuric acid,and the cocrystallizing point of the tertiary mixture were determined. From the experimental results,it is concluded that in order to precipatate more 2,6-DNT,the crystallization process should be carried out at higher temperature for solvent of ethanol,but at lower temperature for solvent of sulphuric acid.