One strain, identified as Streptomyces sp. FX645 which was isolated from the sludge collected in a printing and dyeing mill, had high potency of degradation and decolourisation of azo dye Red 30 (AR30). The microbial degradation mechanism on AR30 by strain FX645 was proposed through analyzing the UV-vis spectra and LC-MS spectra of the degradation products and investigating the variations in the concentrations of the degradation products in the culture. It is suggested that the azo bond of AR30 was iniially cracked by azo reductase to produce 2,6-dichloro- 4-nitrobenzenamine and 2-[(4-aminophenyl)-(2-cyanoethyl) amino] ethylacetate, which then generated several aromatic amine compounds under the actions of nitror4duction, aminoacylation and cyano hydrolysis, respectively.