In this study, the antioxidant activities of different molecular weights (M(w)) and grafting ratios of chitosan-caffeic acid derivatives were investigated. The grafting process was achieved using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDAC) as covalent connector under different conditions such as molecular-weight of chitosan, molar ratio of chitosan and caffeic acid, reaction temperature, pH, and reaction time. The half-inhibition concentrations (IC(50)) of products were calculated by reduction of the 1,1-diphenyl picryl hydrazyl in the radical-scavenging assay and reduction of the Fe(3+)/ferricyanide complex to the ferrous form in reducing power assay. The EDAC showed maximum activity at 3-h, pH 5.0 and room temperature conditions, except high-molecular-weight chitosan in pH 2.0. The products were water-soluble in all pH and showed lower viscosity than native chitosan. The highest grafting ratio of caffeic acid was observed at 15% in low-molecular-weight chitosan. After 5% grafting of caffeic acid into chitosan, the grafting efficiency was increased by decreasing molecular-weight of chitosan at the same conditions. Caffeic acid has main role in the antioxidant activity of products. The maximum IC(50) of radical-scavenging activity (0.064 mg/ml) was observed at the highest caffeic acid containing derivative. Water-soluble chitosan and caffeic acid derivatives were obtained by this study without activity loss. (C) 2010, The Society for Biotechnology, Japan. All rights reserved.