In this work, the morphological and structural behaviors of polyamide 6 (PA6) and polyamide 6.6 (PA6.6) nanocomposites with carbon nanotube (CNT) were investigated using small angle X-ray scattering (SAXS), wide angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC) and optical microscopy. The combined study provides information on CNT nucleation ability and its influence in the crystallinity degree and structure of polyamides. For both nanocomposites with 5 wt% of CNT, an heterogeneous dispersion became apparent, with some agglomerates. CNT acted as a strong nucleation agent and affected the melting and crystallization temperatures and also the spherulitic structure of polyamides. However, the crystallinity degree for both nanocomposites was not significantly changed (25-30%). WAXS showed identical local crystal structure (unit cell) for polyamide 6.6 and modification in the polyamide 6 structure. The lamellar long period of both polyamides was higher than nanocomposites, with low polydispersity values. However, the crystalline thickness remained almost constant and the amorphous thickness increased with the CNT content. From 2.5 wt.% CNT a high value for amorphous lamella was observed, indicating that NTC's were expelled from the crystalline phase and were accommodated in the amorphous regions.