A reactive disperse dye having $alpha$-bromoacrylamide group was synthesized and its dyeing and fastness properties were compared with a disperse dye. The amount of absorbed reactive disperse dye was less on polyester fiber than disperse dye, and was much higher on nylon fiber. When polyester and nylon fiber were simultaneously dyed in a dye pot, nylon fiber absorbed more reactive disperse dye than polyester fiber. The use of carrier or the variation of dyebath pH were not sufficient for same color depth on both fibers. The N/P union fabric could be dyed with the reactive disperse dye and its color fastnesses were good to excellent.