Abstract: Na-montmorillonite (Na-MMT) which was modified by diphenyl methane-4,4'-diisocyanate (MDI) was exfoliated in nanoscale, MDI modified Na-MMT (M-MMT) was prepared. And then add M-MMT into monomer casting nylon 6 system to solve the thermal degradation problem that organic montmorillonite has presented in monomer casting nylon 6 system and improve the heat resisting property of the composites. The modification mechanism and dispersion of Na-MMT were investigated using FITR, GC-MS, XRD and TEM. Heat resisting, mechanical properties and the microstructure of M-MMT/monomer casting nylon 6 composites were investigated by TG, vicat softening, thermal deformation temperature test, mechanical property test and SEM observation. The results show that M-MMT is exfoliated by MDI, which is attributed to the reactions between MDI and the hydroxyl or the water in Na-MMT layers. In the monomer casting nylon 6 system, due to the mensurable initiator and those consumptions by MDI, the molecular mass of the M-MMT/monomer casting nylon 6 cmposites decreases, which leads to the declining mechanical properties, while the heat resisting of the cmposites improve because of the nanoscale effect of M-MMT. The vicat softening temperature increases up to 224℃, and the thermal deformation temperature increases up to 96℃, under loading condition of 1.82 MPa.