Tensile tests on semi-crystalline thermoplastics generate engineering stress-strain curves, which may exhibit non linearity and/or peak stress associated with a striction/necking of the specimen at the macroscopic scale. This work addresses this state of deformed specimen where irreversible strains lead to a variable cross section area along the necked region. Using Synchrotron Radiation Computed Tomography with two high resolutions, 3D images in this region were exploited. The best resolution (1 pixel length = 0.05µm) allowed a better understanding of the morphology of several deformed spherulites within which polar fans arrangement was clearly detailed. For 0.7µm resolution, the longitudinal and transverse elongations of larger number of spherulites were measured. The evolution of the volumetric plastic strain due to cavitation at the spherulitic scale along the necked region was comprehensively analyzed. Volume change at this scale was highlighted, consisting of increase in the case of void growth followed by a decrease (compaction) at large strain due to the collapse of elongated voids. The effects of these results on the establishment of reliable constitutive model are discussed.