The properties of poly(ethylene terephthalate) (PET) filaments embedded with nanosize clay powder were evaluated. The moisture management function and thermal properties of the textile were found to be improved; however, the strength of the filament was lowered with a little increase in elongation. In situ analysis of the crystalline structure with small-angle X-ray scattering and wide-angle X-ray diffraction during the tensile testing showed an unstable change in intensities during the elongation. This phenomenon is related to the earlier rearrangement and breaks down the microstructure in local regions with an increase in strain. This is expected to be due to the low adhesion strength between the matrix and the embedded nanosize clay particles. V C 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46321.