Biocompatible poly(L-lactic acid) (PLA) was successfully covalently grafted onto the convex surfaces and tips of the multi-walled carbon nanotubes (MWNTs) by one step based on in situ polycondensation of the commercially available L-lactic acid monomers. The functional groups in the carboxylic multi-walled carbon nanotubes (MWNT-COOH) showed active enough for participating the polycondensation of L-lactic acid. The resulting PLA-grafted-MWNTs were characterized with Raman spectroscopy, Fourier-transform IR (FTIR), UV-vis, H-1 NMR, thermogravimetric analyses (TGA) and transmission electron microscopy (TEM). Raman, FTIR and H-1 NMR spectroscopies revealed that the PLA was covalently attached to the MWNT. TGA showed that the grafted PLA content could be controlled by the reaction time. The core/shell structures with MWNT as the "hard" core and the PLA polymer layer as the "soft" shell can be clearly seen through HRTEM. (C) 2007 Elsevier Ltd. All rights reserved.