Abstract: 1,6-hexamethylene diamine (HMD) was grafted onto multi-walled carbon nanotubes (MWNTs) via acid-thionyl chloride route. The characterization of MWNTs was carried out by Fourier transform infrared spectrum, thermal gravimetric analysis, field emission scanning electron microscopy and Raman spectrum. The results show that HMD is covalently bonded on the MWNTs after activating by thionyl chloride. During this process, the compact stacking microstructure induced by acid treatment is deaggregated. The apparent specific volume of functionalized MWNTs is even larger than that of the pristine MWNTs. The chemical processing of MWNTs modification was analyzed theoretically in detail and the mechanism of microstructure transformation was speculated. It is proposed that HMD molecules inserted in different carbon nanotubes and break the hydrogen bond structure formed in acidification process. Therefore the dense structure derived from acidification is modified, leading to a less compact stacking and larger apparent specific volume of MWNTs.