Structural and morphological behavior under stress-strain of polypropylene/multi-walled carbon nanotubes (PP/MWCNTs) nanocomposites prepared through ultrasound-assisted melt extrusion process was studied by means of optical microscopy, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, small angle X-ray scattering (SAXS), and wide angle X-ray scattering (WAXS). A high ductile behavior was observed in the PP/MWCNT nano-composites with low concentration of MWCNTs. This was related to an energy-dissipating mechanism, achieved by the formation of an ordered PP-CNTs interphase zone and crystal oriented structure in the undeformed samples. Different strain-induced-phase transformations were observed by ex situ SAXS/WAXS, characterizing the different stages of structure development during the deformation of PP and PP/MWCNTs nanocomposites. The high concentration of CNTs reduced the strain behavior of PP due to the agglomeration of nanoparticles. A structural pathway relating the deformation-induced phase transitions and the dissipation energy mechanism in the PP/MWCNTs nanocomposites at low concentration of nanoparticles was proposed. (C) 2014 Wiley Periodicals, Inc.