Ionizing radiation is a promising method for dye degradation or textile coloration using commercial azo dyes and small molecular weight organic dyes. Thus, the stability of the molecular structure of an azo dye is important under ionizing radiation. Disperse Blue 79, as an example azo dyes, was irradiated with gamma rays or electron beam (EB) to investigate the radiation-induced effects on the molecular structure. Ultraviolet visible spectroscopy (UV–Vis), nuclear magnetic resonance (NMR) spectra analysis, and mass spectrometry (MS) studies indicated that acetoxy and methoxyl were easily cleaved on the irradiation of the aqueous dye solution but retained a stable structure on the irradiation of the powder form. Gamma rays and EB showed similar effects on the decomposition process. Chromaticity changes using the Lab* method showed that the dye tued to dark yellow and the value of b*of the irradiated dyes increased with the increasing absorbed dose, which indicated that Disperse Blue 79 could be partly decomposed in an aqueous solutionwith an absorbed dose of 10 kGy. Furthermore, the results demonstrated that the chemical stability of the Disperse Blue 79 under ionizing radiation are different in its powder form with the dye in the aqueous solution.