Multi-wall carbon nanotube (MWCNT)/nylon-6 composites made by in-situ polymerization and subsequently modified by treatment at 1.0 GPa (or 1.7 GPa) and 530 K have been studied by WAXD, DSC and NMR. The pressure treatment gives an amorphous to crystalline transformation where the crystallinity increases from ∼31% to as much as ∼58% concurrently as the nylon-6 crystals increase in size and attain a preferred orientation relative to the applied pressure. A composite of 2.1 wt% purified MWCNT in nylon-6 shows significantly higher melting temperature than neat nylon-6 after identical pressure treatments. The improved thermal stability of the composite is attributed to crystal growth in the presence of reinforcing MWCNTs. The NMR spectrum of a pressure treated composite is similar to that of nylon-6 single crystals, which suggests a reduction of crystal boundaries after treatment, but there is no indication of covalent bonds between the nylon-6 chains and the MWCNTs.