One bath dyeing method by disperse dyes/vat dyes for polyester/cotton blended fabrics was studied. Under vat dyeing condition, the effects of alkali, reducing agent and neutral salt on the stabilities of disperse dyes were investigated by 상e reflectance measurement and TLC. Cotton staining and washing fastness of disperse dyes were also measured. The disperse dyes on the fabric showed little reflectance change. But the disperse dyes in dyebath showed 40- 60 f decomposition with sodium hydroxide and hydrosulfite, and the degrees of those were varied by their chemical structure.