Biodegradable poly(L-lactide) (PLLA) has been covalently grafted onto the surface of magnetic multi-walled carbon nanotubes (m-MWCNTs) by in-situ ring-opening polymerization of lactide. The content of grafting PLLA can be controlled by adjusting the feed ratio of monomer to m-MWCNTs. FT-IR and Raman spectroscopy confirm that PLLA have been covalently attached to the sidewalls of m-MWCNTs. Thermal gravimetric analysis (TGA) indicates that the composites of PLLA grafted m-MWCNTs have a polymer weight percentage of ca. 25.6-33.7 wt%. The scanning eletron microscopy (SEM) and transmission electron microscopy (TEM) are utilized to image the PLLA grafted m-MWCNTs, showing relatively uniform polymer layer coated on the surface of m-MWCNTs. The composites of PLLA grafted m-MWCNTs exhibit superparamagnetic behavior at room temperature and are aligned under a low magnetic field. Crown Copyright (C) 2008 Published by Elsevier Ltd. All rights reserved.