Effects of chemical structure of disperse dyes applied to conventional and microdenier polyesters on the dyeing property and washfastness were studied. It was found that washfastness of dyed polyester fabric is closely related to the degree of thermomigration of disperse dye during heatsetting. The bulky disperse dye, which was synthesized by substituting two acetoxy groups of C.I. Disperse Red 82 with two benzoxy groups, showed almost the same amount of absorbed dye by the microdenier polyester as C.I. Disperse Red 82, but the degree of thermomigration was low and subsequent washfastness was excellent. The high grade of washfastness of the bulky disperse dye might be caused by the increased dye-fiber interaction and the reduced mobility.