The synthesis of maleic anhydride grafted poly(lactic acid) (MAH-g-PLA) was successfully achieved by reactive extrusion using a twin screw extruder. The effects of initiator (dicumyl peroxide) and maleic anhydride contents on grafting efficiency were investigated. By reactive extrusion method, grafting reaction was achieved after confirming by FTIR and NMR. FTIR spectra showed absorption bands at 1,759 cm⁻¹ and 3,300-3,600 cm⁻¹, corresponding to carbonyl ester group and OH stretching of pendant carboxylic group. From NMR spectra of grafted PLA, chemical shifts at 3.9 and 4.7 ppm were observed which were responsible for the methylene protons of maleic anhydride moiety. The quantitative analysis by back titration revealed that the grafting degrees of 1.6% and 3.29% were obtained for 2.5 wt% and 5 wt% maleic anhydride, respectively. On the other hand, the amount of dicumyl peroxide had insignificant effect on the degree of grafting when compared to maleic anhydride. However, GPC results showed that the molecular weight of PLA substantially decreased due to the side reaction of β-Scission.