Effects of dyeing temperature, dyeing time, concentration of sodium hydrosulfite, and addition of anti-overreduction agent, on the reduction behaviors of the two anthraquinone vat dyes were investigated. In the case of Indanthrene Blue BC (C.I. Vat Blue 6) the overreduction appeared apparently as the dyeing temperature and the amount of reducing agent increased. Addition of the anti-overreduction agent such as glucose depressed the occurrence of overreduction of Indanthrene Blue BC to some extent. Reduction behavior of indanthrne Blue CLF(C.I. Vat Blue 66) was not affected by the dyeing temperature and the amount of sodium hydrosulfite, suggesting that this dyestuff was quite stable in reductive conditions due to the strong intramolecular hydrogen bond between NH and O atom of the carbonyl group.