Abstract: By means of ESR and chromatographic analysis,it was confirmed that the azido-nitrene and dinitrene which produced by the photo-decomposition of 4,4'-diazidobiphen-yl were triplets,the photo-decomposing reaction carried on in two steps.But the intermediate which produced by the thermo-decomposition of 4,4'-diazidobi-phenyl was singlet dinitrene only. So the mechanism of the thermo-decomposition of 4,4'-diazidobiphenyl was totally different from that of the photo-decomposition.