The natural dyeing of cotton fabric with gardenia blue powder was studied. Cotton fabrics were treated with chitosan in order to increase K/S values and colorfastness. K/S values were increased with increasing the concentration of gardenia blue powder, dyeing time and temperature, and the lower pH of dye bath. In case of chitosan finishing, K/S values were increased, and ${lambda}_{max}$ shifted to 600nm at 400nm. The wash fastness was improved, but the abrasion fastness was lowered in the case of wet rubbing.