Multi-walled carbon nanotubes bearing hydroxyl group were modified with toluene diisocyanate (TDI) and stabilized with caprolactam. The functionalized carbon nanotubes were used to prepare monomer casting polyamide 6(MC nylon 6)/carbon nanotubes nanocomposites. The results of FTIR proved that isocynate groups have been incorporated into carbon nanotubes successfully. XRD results showed that the addition of the functionalized carbon nanotubes has no significant influence on the crystal structure of MC nylon 6, whereas the untreated carbon nanotubes could hinder the growth of α2-crystal of nylon 6; with the loading of 0.3 Wt% of the functionalized carbon nanotubes, the crystallinity degree of the nanocomposites is almost equal to that of pure MC nylon 6, whereas as the loading increased to 0.5 Wt%, the crystallinity degree of MC nylon 6 decreased. DSC revealed that carbon nanotubes in MC nylon 6 acted as effective nucleation agents. The peak temperature of crystallization for MC nylon 6 increased by 10 ℃ from 173.4 to 183.5 ℃, and the temperature range of crystallization decreased, indicating that the carbon nanotubes act as heterogeneous nucleation agents for MC nylon 6.