Poly(L-lactic acid) (PLLA) nanocomposites reinforced with graphene oxide (GO) or GO grafted with PLLA (GO-g-PLLA) were prepared by solution blending and then compression molding method, where GO-g-PLLA was synthesized via one step based on in situ polycondensation of the L-lactic acid monomers initiated by lyophilized GO. The average thickness of GO was increased by about 2 nm and the weight percent of grafted PLLA chains in obtained GO-g-PLLA was as high as 30.8%. It was clearly evidenced that the functionalization of GO with this method enhanced its dispersion and interfacial interactions with the PLLA matrix and thus improved the final thermostability and mechanical properties of nanocomposites. The flexural and tensile strength of PLLA/GO-g-PLLA nanocomposites were increased by 114.3% and 105.7%, respectively, compared with neat PLLA. (C) 2013 Elsevier B.V. All rights reserved.