Polylactic acid (PLA) was successfully covalently grafted onto multi-walled carbon nanotubes (MWCNT) by microwave-assisted polymerization of lactide monomers. The final products MWCNT-g-PLA were characterized with Fourier-transform IR (FTIR), Raman spectroscopy, thermogravimetric analyses (TGA) and transmission electron microscopy (TEM). The results indicated PLA chain was covalently attached to the MWCNT. The grafted PLA was uniformly coated on the surface of MWCNT with a layer thickness of 2 ~ 6 nm. The grafted PLA content could be controlled by microwave irradiation time and the concentrations of reactant. The product with 60.5% grafted PLA content can be synthesized in one hour.