Automatic belt tensioner for stabilization of Engine accessory belt system is made increase its life, decrease noise and vibration and increase its commercial value. Therefore, it is important finding test mode to assess life of automatic belt tensioner. However, measurement and analysis test result in practical test condition on real road needs requiring much test time and finance. In this study, there is analyzed and established test mode which is loaded measured equivalent damage in real working condition, compared test condition of pre-established test mode and FTP- 75 mode of fuel efficiency test and verified advantages and disadvantages of each mode. Because FTP-75 Mode is for measuring fuel efficiency related with engine, applying FTP-75 mode is estimated suitability for test of engine accessory. Comparison and assessment of damage is investigated structure and pulley part. Equivalent damage is calculated with Palmgren-Miner Rule and representative stress method, test condition of automatic belt tensioner is investigated. This investigation is regarded as being useful to life evaluation of both automatic belt tensioner and other engine accessory component.