A novel reactive monoazo dye, which can give to cellulose fiber materials a deep orange to scarlet color superior in build-up property and chlorine fastness, represented by a free acid of the formula, ##STR1## wherein D is a phenyl group substituted with 1 to 3 substitutes selected from methyl, ethyl, methoxy, ethoxy, halogen, acetylamino, propionylamino, benzoylamino, succinylamino, maleinylamino, nitro, suflonic acid and carboxyl groups; R.sub.1 is a hydrogen atom, or a methyl or ethyl group; R.sub.2 is an alkyl group having 1 to 4 carbon atoms which is unsubstituted or substituted with a hydroxy, cyano, alkoxy, halogen, carboxy, alkoxycarbonyl or sulfonic acid group; Z is a hydrogen atom or a sulfonic acid group; A is a phenylene residue which is unsubstituted or substituted with 1 or 2 substituents selected from methyl, ethyl, methoxy, ethoxy, chlorine, bromine and sulfonic acid groups; or a naphthylene residue which is unsubstituted or substituted with one sulfonic acid group; and X is --SO.sub.2 CH.dbd.CH.sub.2 or --SO.sub.2 CH.sub.2 CH.sub.2 Y in which Y is a group which is splittable by alkalis.