N-Picryl-p-toluidine (I) and N-(2,4-dinitrophenyl)-o-anisidine (II) crystallize in two modifications, orange and red forms. The orange forms of both compounds transform to the red forms on heating, while the red forms do not change thermally. The thermal transformation of I from the orange form (Io) to the red (Ir) takes place through the liquid phase. In the case of II, the transformation from the orange form (IIo) to the red (IIr) occurred in the solid state over the wide temperature range, 110 similar to 158 degrees C. In order to investigate the mechanism of the phase transformation between these polymorphs, we carried out the observation on the hot-stage with the microscope, the X-ray structure analysis, measurement of temperature dependent powder diffractions, and calorimetry. We also investigated the preliminary study on the structure at high temperature.