Repeated loads are applied to mooring ropes of a vessel under mooring. Occasionally, their breakage may happen and may lead to a serious accident. The authors considered that causes of the mooring rope's breakage are the rope repeated pulling and the resulted heat. In the tensile tests, the temperature of rope rises when a rope is pulling. It is observed with an infrared camera. At first the authors considered that the area under Load-Stroke Curve indicates the energy that the rope gets and the energy transfers to the heat and then the temperature of rope rises. The authors guessed that the characteristic of rope changes due to the heat, with the result that a rope becomes weak. However, from Energy Conservation Law, the consideration was very illogicality. Then the authors consider that a friction in the rope generates a fever and the fever makes the temperature of the rope to rise. In this research, the force in the mooring rope under tensile test is investigated with a subminiature pressure sensor and a non-contact infrared thermometer. The obtained results are summarized as follows; (1) The pressure in the rope is measured with the pressure sensor. The force on the sensor in the rope is about 4% of the tensile load. (2) The temperature rise of mooring ropes under repeated load was investigated continuously. (3) The numerical calculated results show the almost same tendency as the measurement ones.
