Thermomigration is the most important factor deteriorating thewashfastness of polyester fabric dyed with disperse dyes, and this phenomenon becomes more evident as the denier of fiber decreases. Factors affecting thermomigration are known to be fiber structure, chemical structure of dye, amount of dye on fiber, and fiishing conditions. Among them, the effect of chemical structure of disperse dye on the washfastness was studied. The bulky disperse dye, which was synthesized by substituting two acetyl groups of C.I. Disperse Red 82 with two benzoyl groups, showed almost the same amount of absorbed dye by 0.5 denier micropolyester fabric as C.I. Disperse Red 82, but the degree of thermomigration was low and subsequent washfastness was excellent. The high washfastness of the bulky disperse dye may be due to the increased dye-fiber interaction such as van der Waals force and the decreased diffusion rate.