Investigation was made on the thermal stress and loss tangent of nylon 6 fibers oriented at different draw ratios and heat set at different temperatures and the following results were obtained. 1. Thermal stress of nylon 6 fibers can be divided into two components, one is contributed from the amorphous regions, and the other from the crystalline regions. 2. Thermal stress of nylon 6 fibers contributed from the crystalline regions tends to increase with draw ratio irrespective of heat setting temperature. 3. Thermal stress contributed from the amorphous regions decreases with heat setting temperature. 4. Max. loss tangent decreases with the increase of draw ratio and increases with heat setting temperature, while max. loss tangent temperature increases with draw ratio and decreases with heat setting temperature. 5. Max. loss tangent and its temperature are linearly correlated with negative slope.