This work reports an in situ WAXS and SAXS investigation, under X-ray synchrotron source radiation, on the structural evolution during solid-state uniaxial deformation of poly(ethylene terephthalate) (PET) nanocomposites with 0.3 wt % of 3D nanoparticles [nanotitanium dioxide (TiO2) and nanosilica (SiO2)]. Good dispersion and average agglomerate sizes of nanoparticles of about 80 nm for both nanocomposites were revealed by transmission electron microscopic characterization. The influence of the nanofillers on the deformation-induced phase's formation and their evolution along the stretching process were compared with respect to the neat PET. WAXS results indicated that the structural evolution of all samples passes through three main stages, with evolution of amorphous phase into mesophase, a rapid increase of molecular orientation, and the formation of a periodical mesophase (PM). The incorporation of the nanofillers promoted a higher fraction, and an earlier formation, of PM during stretching when compared with pure PET. Furthermore, the presence of TiO2 nanoparticles in the PET matrix resulted in the earliest formation and the highest amount of PM and the retardation of crack growth and bigger voids when compared with PET/SiO2 nanocomposite. A multiscale structural evolution mechanism is proposed to interpret these results. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 39752.