The mehcanism of azo dyes decolorization by Candida sp. MK-1, Aeromonas sp. B-5 and Actinobacillus sp. B-11 were analyzed. The maximal decolorization activity was observed at pH 7.5 and 30℃ on Candida sp. MK-1, at alkaline and at 35℃ on Aeromonas sp. B-5 and Actinobacillus sp. B-11. The HPLC analysis of the supernatant of the Acid Red 27 detected in the blank. The retention time of this peak matched that of a reference standard compound of 4-amino-1-naphthalenesulfonate, produced by reductive cleavage of Acid Red 27. The decolorization of azo dyes with cell free extract of Candida sp. MK-a was promoted by the addition of several coenzymes or lawsone. The remarkable promotion of decolorization was observed by the addition of glutamate dehydrogenase with α-ketoglutarate and NH4+. Therefore, it was suggested that Candida sp. MK-1 azoreductase catalyzed decolorization of azo dye by NADPH dependent reductive cleavage.