In order to investigate the increased damping for a moored semi-submersible during lowfrequency motions in waves, free oscillation tests were conducted both in still water and regular waves. Test results show that the wave effects as well as the viscous effects have on the low-frequency damping and it seems that the interaction effects among columns play an important roll in the increased damping due to wave effects in short waves. Calculations and experiments in regular wave groups were also compared to investigate the effect of the increased damping due to waves on the low-frequency motions.