<p>Natural dye is known to have poor fastness qualities and weak shades in colouring fabrics, and to improve fastness qualities and strength of the shades a mordant is needed. This study therefore aims to investigate the effect of different mordants concentration on the colour strength of the ethyl acetate extract of Phyllanthus muellerianus dyed wool. The aqueous filtrate of acetone and water (50/50 v/v) extract was partitioned successively with n-hexane, chloroform, ethyl acetate and n-butanol. The ethyl acetate fraction in ethanol was separated into its various fractions using thin layer and column chromatography that afforded one compound characterized spectroscopically. The sample absorbs at ultra violet and visible region and the FTIR spectra revealed the presence of carbonyl, resonance, and conjugation. GC-MS analysis characterized the base peak and molecular weight of the compounds is 449.11. The 1H NMR spectra of the extract is dominated by the resonances from cyanidin 3- glycosides (cy 3 – gly). Wool (Cashmere wool) swatches were dyed at acidic pH using a cyanidin glycoside dye with different types of mordents at varied concentrations between 2.5, 5, 7.5, 10, 12.5 and 15%. The results of the effect of mordant concentration on the color strength obtained showed that the color strength was optimal for 7.5 -10 % K2Cr2 O7 and 5-7.5 % CuSO4 as mordants concentration. Copper sulphate (CuSO4) mordanted wool fabric has better and more stable colour strength that increases with increase in mordant concentration than potassium dichromate (K2Cr2O7).</p>