Abstract: The microstructures and mechanical properties of the latest T800 elastic yarn were studied in this paper. Cross section and longitudinal surface were observed and analyzed by SEM. Nicolet 5700 was used to test the IR spectrogram of T800 filament which indicated the elements of I、Cl、and N were included but the regular PET macromolecular chain contains only three elements C、 H、O.Then we employed DSC to test the fiber's crystallinity which indicated that the fiber macromolecule, matrix fibril arrangement wasn’t very regular, the gap was larger.Meanwhile,testing and comparing the tensile strength and elasticity properties of the three kinds of fibers,the smaller the fineness,the better the tensile strength and elasticity.This study has a certain theoretical and practical guiding significance for the raw material selection for new elastic fabric and the research for elasticity properties

Key words: elastic yarn, cross section, longitudinal structure, crystallinity, mechanical properties