It is well-known that the noise and vibration from the torque change of the engine and the inertia change of its accessories (alternator and water pump) have severe effects on both the quality and the life expectancy of the engine belt system. An automatic belt tensioner is generally used not only to reduce the noise and vibration of such belt system, but also to enhance the life cycle. In the present research, measurement of the user running conditions and the data analyses were performed to try to derive an evaluation method of life cycle of such an automatic belt tensioner system. As a part of results, a running condition for the evaluation was suggested from the study of conditions loaded on an automatic belt tensioner. This running condition was divided into two parts, the Structure part and the Pulley one, and suggested separately for the accelerated life-test of the system. As the other part of the results, an accelerated life-test condition was derived using Palmgen-Miner rule for structure part. And a modified accelerated life-test condition, which uses temperature as an acceleration factor, was suggested for pulley part.