To obtain a crystalline monoazo disperse dye which can dye satisfactorily at high temps. under severe conditions by selecting one which has high- strength peaks at a plurality of specified angles among peaks appearing in a specified range of diffraction angle (2θ) in X-ray diffraction with Cu-Kα ray.

This crystalline monoazo disperse dye is represented by the formula and has high-strength peaks at diffraction angles (2θ) of 5.6° and 10.6° among peaks appearing in a range of diffraction angle of 5° to 25° in X-ray diffraction with Cu-Kα ray. A hydrophobic fiber material (e.g. polyester fibers produced by spinning at a speed of 6,000 m/min or higher) can be dyed homogeneously in a deep color by dispersing the dye in an aq. medium in the presence of a dispersant (e. g. a higher alcohol sulfuric ester) in a wt. of less than 3 times the dye and dyeing the fiber material at a dyeing temp. of 125-140°C in a dye bath ratio of 15 or lower, if necessary in the presence of an acidic material (e.g. acetic acid). Hydrophobic fibers include polyester fibers, acetate fibers, and mixtures of these fibers with natural fibers.