A series of octadecylamine (ODA) grafted multi-walled carbon nanotubes (MWNT) were prepared and characterized by NMR, FTIR, DSC and XRD. It is revealed that the ODA chains grafted on MWNT are partially crystallized, though the packing structure is not as perfect as the pure ODA. It is believed that ODA prefer to react at some active sites, such as the tube-ends and the defects on the sidewall of MWNT. A quantitative NMR technique was developed to determine the weight percent of ODA in modified MWNT. The relationship between the content of ODA and the solubility of soluble MWNTs is discussed. (C) 2003 Elsevier Science B.V. All rights reserved.