In situ anionic ring opening polymerization is used to prepare monomer casting polyamide 6 (MCPA6)/carbon nanotubes (CNTs) nanocomposites, whereby water is used as auxiliary dispersing agent of hydroxyl functionalized multiwalled carbon nanotubes (MWNTs-OH) and epsilon-caprolactam (CL) monomer. The MWNTs-OH were dispersed homogenously in MCPA6 matrix when being observed through transmission electron microcopy. The well dispersed MWNTs-OH existed at the center of many radial texture phases in MCPA6 matrix. Polarizing microscope analysis showed that these radial texture phases were MCPA6 spherulitic crystallities. Differential scanning calorimetry analysis revealed that the crystallization temperature of the MCPA6/MWNTs-OH nanocomposites had been improved by adding only 0.2 wt % MWNTs-OH when compared with pure MCPA6. The influence of MWNTs-OH on the thermal stability of MCPA6 under nitrogen and air environments was also investigated by thermal gravimetric analysis (TGA). (c) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 1278-1285, 2009