Slow motion forced oscillation tests in regular waves are carried out concerning slow drift oscillation of a floating body moored in irregular waves. Dependences of waveheight, wave frequency, motion amplitude and motion frequency on both damping and added mass are examined. Results show that wave drift damping is not necessarily proportional to square of waveheight. Motion velocity is also a very significant parameter to the wave drift damping, and velocity ratio of a wave orbital motion to a body motion is important as well. Wave frequency affects the wave drift damping as well known. The most significant parameters to the added mass are K _c number and velocity ratio of a wave orbital motion to a body motion. The added mass is affected by waves only in case of small K _c number.