Compared with conventional adsorption-based coloration, the photoreactions of dyes such as photo-copolymerization and photo-crosslinking under UV irradiation can be employed for the coloration of textiles, which can be carried out without salt addition at room temperature. C.I. Reactive Black 5, a homo-bifunctional reactive dye containing two sulfatoethylsulfone groups, is used as a photo-reactive dye for wool fibers. Upon UV irradiation, the photo-reactive dye was grafted onto wool fabrics without photoinitiators. Since the disulfide bonds in the cystine residues of wool can be easily photodecomposed to active thiyl radicals which initiate the polymerization, the dye can be polymerized to an oligomeric dye of a degree of polymerization of 12 or more. The grafted fabrics reached a grafting yield of 2.3% o.w.f. and a color yield (K/S) of 18.2 by the photografting of an aqueous dye concentration of 9% using a UV energy of 25J/$cm^2$. Furthermore, the photochemically dyed wool fabric showed higher colorfastness properties to light, laundering and rubbing comparable to conventional reactive dyeing.