The symmetric-type stockbridge damper was installed in South Korea for decades in order to control the aeolian vibration of transmission power line and the replacement of the installed stockbridge damper was scheduled only if it is found the failure of the stockbridge damper by visual inspection. However, few dynamic characteristics of the installed stockbridge damper was not conducted so far so that the performance of the in-service damper device was not guaranteed up to now. In this paper, the evaluation of the stockbridge damper was done in field condition whether the expected controllability of the stockbridge damper was still valid or not. The comparison of the acceleration response of transmission power lines are processed for two cases, with and without damper, for the same wind situations. The evaluation of the installed symmetric-type stockbridge damper was conducted from the measured acceleration responses.