The correlation of thermal properties and nanostructure of nylon 6 (denoted PA6) reinforced with polymer nanoparticles (denoted PNP, size~8 nm) has been investigated. PNPs are highly crosslinked acrylic-based polymers synthesized by the Rohm and Haas Co. PNPs and those grafted with maleic anhydride (denoted PNP-g-MA) were each one dispersed into a commercial PA 6 matrix by melt extrusion, at a concentration of 3 wt%. Thermal analysis showed that the PNPs increased the thermal stability of PA6 and reduced the melting and crystallization temperatures as well as the enthalpy. Small-angle light scattering showed that the PNPs crystallize into a spherulitic morphology, typical of PA6. Isothermal crystallization studies showed that the PNPs act as nucleating agents, accelerating the rate of crystallization. Wide-angle X-ray scattering showed that the composites crystallize in the α-form, and the PNPs reduced the degree of crystallinity, in agreement with thermal analysis results. The smaller degree of crystallinity was also reflected in the long range spacing, it was also reduced by the presence of the PNPs as measured directly by small-angle X-ray scattering.