The loss tangent of the composite is measured but not systematically, for the reason of difficulties in the method of measurement and the vagueness in the measured values when including cracks. Recently, the direct measurement is possible to make use of the micro-computer and the A/D-converter for variation of stress-strain loop during fatigue without the vibration-and torsional-method. In this paper, some conclusions are reported from the measurements of loss tangent for the composites during fatigue as follows: 1) The temperature rise of the specimen is caused by the accumulated energy due to a half of area in the hysteresis loop during fatigue. 2) The change in loss tangent with temperature depends on composite structure. 3) The increase of loss tangent in the cloth FRP is larger than in other composites during fatigue: this depending on differences in composite structure.
