Damping for ship vibration is large as compared with that calculated from material damping by the theory of flexural vibration of beams; in most cases the former is ten times as large as the latter. Hitherto the internal friction of materials is considered to play the most imporcant role for damping in the ship vibration. However, the friction of surrounding water and the energy dissipation due to cargoes may have a marked effect on damping. In this paper, the damping forces of these kinds are investigated theoretically and experimentally, it is concluded that the discrepancy between experimental and calculated values of damping in ship vibration is due to frictional forces of water and cargo.