The compatibilization effects provided by ammonium terminated PMMA (PMMA-t-NH3+) on monomer casting polyamide6 (MCPA6)/clay(pristine sodium montmorillonite) nanocomposites were studied in this article. PMMA-t-NH3+ used in this study was prepared by radical polymerization using 2-aminoethanethiol hydrochloride as chain transfer agent. MCPA6/clay/PMMA-t-NH3+ nanocomposites were prepared by in situ anionic ring-opening polymerization of epsilon-caprolactam. X-ray diffraction and transmission electron microscopy plus rheological measurement were used to characterize those nanocomposites. The results indicated that PMMA-t-NH3+ would be a good compatibilizer for this system. With PMMA-t-NH3+ content increasing, a better dispersion of clay was successftilly achieved in the MCPA6 matrix. Furthermore, analysis using differential scanning calorimetry indicated that well dispersed clay layers limited the mobility of the MCPA6 molecule chains to crystallize, reduce the crystalline degree, and favor the formation of the gamma-crystalline farm of the MCPA6 matrix. (C) 2008 Wiley Periodicals, Inc.