Poly(ethylene phthalate)(PET)/nano-antimony doped tin oxide(ATO)composites prepared via in situ polymerization were spun into fiber by the melt spinning process.Comparing with the neat PET,the tenacity of PET/ATO hybrid fibers was improved,and PET/ATO hybrid fibers had a lower elongation at break by adding nano-ATO.The ATO nanoparticles improved the thermal stability of PET fiber.The WAXD and DSC results suggested that ATO nanoparticles increase the degree of crystallinity of PET acting as the nucleating agent,which prohibited the thermal shrinkage of PET fiber.