Dyeing cotton fabric with direct dye (C. I. Direct Blue 15) by redox sytem of ammonium persulfate as an oxidant and glucose as reductant was studied. It was found that covalent bond between dye and cellulose molecule can be formed by free radical produced by the redox system in the dye bath, which enhanced significantly the color strength. The retained color strength after DMF extraction was much better in the presence than in the absence of the redox sytem. The optimum dyeing condition was 0.028 mol/$ell$(APS/Glucose each) of redox concentration, 65$^{circle}C$ of dyeing temperature and 60min of dyeing time. The color variation on the dyed sample had not been observed as a result of fixed ${lambda}_max$.