Abstract: A crystalline form of the dye represented by the following formula (1): ##STR1## characterized by an X-ray diffraction pattern having a strong peak at the angle of diffraction (2.theta.) of 23.2.degree. and relatively strong peaks respectively at the angles of diffraction (2.theta.) of 8.3.degree., 12.3.degree., 20.8.degree., 21.9.degree., 25.0.degree., 27.1.degree., 29.2.degree. and 30.8.degree. according to X-ray powder diffractometry through the use of a Cu-K.alpha. line and a process for preparing the same by heat-treating a heat-instable crystalline form of the dye represented by the formula (1) in water, water containing a water-soluble organic solvent, or water containing an anionic or nonionic surfactant at a temperature of 40.degree. C. or above.It withstands dyeing at high temperature, e.g. 130.degree. C. or higher, to provide dyed fibers having excellent fastness properties.