In this study, basic research of small semi-active rotation damper in which friction coefficient is controlled by utilizing ultrasonic vibration is dealt with. Experimental rotational friction damper system using ultrasonic brake is introduced as a testing mechanism. Maximum (rotational) coefficient of static friction and coefficient of (rotational) dynamic friction are identified thorough experimental analysis. In this paper, difference in dry and wet friction coefficient variation is investigated. It is shown that the semi-active control does not work in wet friction.