In order to confirm the reduction of vat dyes used in the experiments the leuco potential of each vat dye was measured and the minimum amounts of alkali and reducing agent needed for complete reduction of vat dyes were determined. In addition, the rate of the reduction of vat dyes was measured in various dye bath conditions and the levelling property of vat dyes on cotton in the early stage of dyeing was investigated. It is concluded that both the degree of aggregation of a vat dye and the amount of a reducing agent added in dyebath influence the rate of the reduction of vat dye which have close relation-ship with the levelling property of a vat dye on cotton fabric.