This work investigates the solid state uniaxial stretching of neat polyethylene therephthalate, PET, and its montmorillonite, MMT, nanocomposites (0.3 wt % of MMT particles with different initial agglomerate sizes) showing intercalated and tactoid morphologies, followed by in situ WAXS and SAXS experiments under an X-ray synchrotron source. The distinct nanocomposite morphologies were assessed by WAXS and transmission electron microscopy. The in situ WAXS experiments during stretching evaluated the evolution of phase's mass fractions and the average level of molecular orientation upon uniaxial deformation, and the in situ SAXS experiments assessed the evolution of craze-like structures and void sizes. Multiscale structure evolution models are proposed and compared for neat PET and its nanocomposites. Main global mechanisms are identical although with distinct evolutions of phase mass fractions. Also craze-like/voids structures evolve with distinct sizes. Intercalated MMT morphology induces an earlier formation of periodical mesophase, a retarded widening of craze-like structures and the smallest void sizes.(c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013