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A winding machine with heat treatment was newly developed to strengthen ramie yarns. During the treatment process, ramie yarn in normal or wet state was wound on a winding machine and passed continu- ously through a heater at 100 and 150 � C, respectively, with different winding speeds and tensions. Higher tensile strength and stiffness of ramie yarn was achieved after heat treatment on wet yarn and winding speed had a sig- nificant influence on the tensile properties of yarns. How- ever, a little decline in tensile strength was found for ramie yarns after heat treatment in normal state. This implies that the water-swollened structure of ramie yarn during the heat treatment is crucial in strengthening yarns. In the case of heat treatment on wet yarns, the effect of winding tension on the tensile properties of yarns was studied. It was found that the tensile strengths and Young's moduli of ramie yarns first increased and then reached equilibrium as the winding tension was increased. The crystallinity calculated from X-ray diffraction dia- grams showed a slight decrease in heat-treated ramie yarns whereas the crystalline orientation factors had no appreciable change. It was considered that the improved effect was related to the more oriented molecular chains in amorphous region and optimized yarn structure. V C 2010