Chitosan (Ch), a natural polysaccharide derived from chitin is a widely used raw material in biomaterial research. Modification of chitosan using maleic anhydride (MA) have been done by several researchers with dimethyl sulfoxide (DMSO) and acetic acid as a common solvent. In the present work the chitosan was grafted with maleic anhydride using acetic acid and water as a solvent toward ecofriendly functionalization process. In addition, proposed mechanism of this grafting was further studied. The total acidic content and conversion of grafted films were calculated with varying concentration of MA: Ch ratio and temperature. The success of chitosan grafting with maleic was verified by Fourier Transform Infrared Spectroscopy (FTIR) analysis. FTIR analysis showed the appearance of new ester group and shifting of amide group from FTIR spectra which indicates that the esterification and amidation occurred in this grafting process.