The heat generation of nylon6 filaments by drawing was measured with a thermographic apparatus. The effect of heat on fracture was discussed based on the observation of fractured surfaces by an electron microscope. In a simple elongation, the surface temperature of filaments rose 40-50°C from room temperature, and the temperature of fractured surfaces rose 90-115°C. The heat generated on drawing was due to the friction between polymer chains. The higher temperature of the fractured section than that of the filament surface was attributed to the heat loss with convection and radiation (32°C), the heating due to rapid retraction after breaking (10-20°C), and the heating due to local extension near the breaking point (3-5°C). Round bumps and fine strings were observed on the fractured surfaces. They seemed to be due to softening and melting by the generated heat, which suggests that the partial temperature in the micro-region rose over the melting point. In the elongation of knotted filaments, the surface temperature of knots rose 50-60°C from room temperature, and the temperature of the fractured surfaces rose 100-140°C. The morphology of fractured surfaces of knotted filaments was fundamentally the same as that of simple elongation, and many fine strings were observed. In the plastic deformation of nylon filaments elongation energy was spent in the deformation of the micro-structure and the accompanying heat. The temperature rise after breaking was founded to be smaller than those of brittle polymers.