Variations of the concentration of sodium hydrosulfite used as a reducing agent in vat dyeing in aqueous solution and in dyeing solution, and during dyeing were investigated. Concentrations of sodium hydrosulfite in all cases were considerably affected by the amount of air in dyeing machine. The concentration of the reducing agent in aqueous solution under closed system decreased with temperature and time, but that under the purge of nitrogen gas did not change with temperature and time, but that under the purge of nitrogen gas did not change with temperature, time and the amount of sodium hydroxide. The decomposition behaviors of the reducing agent in dyeing solution and during dyeing were similar to those in the aqueous solution. The concentration of sodium hydrosulfite in dyeing solution decreased with increase in the amount of dye, the leuco potential of dye and the number of keto groups in dye molecule.