Excessive yaw motion of a turret moored vessel was examined in this paper. The vessel was moored by an internal turret and catenary mooring lines. The experiments were conducted in regular waves. Turret moored vessel can oscillate from side to side or move to one particular direction, and eventually exhibit large yaw motion. The results showed that the excessive yaw occured for specific condition, ratio of wave lengths and vessel length. It was found that turret moored floating vessel was unstable for regular wave period from 14 to 18 sec and excessive yaw occurred to maximum 50 deg. The time series, trajectory, phase plot and qualitative analysis are performed. The analysis showed that the results of experiments agreed with the that of analytic method and the excessive yaw could be predicted by the stability analysis.