Errors of a vessel's position by radar observations has not been determined sufficiently, because we do not have a high accuracy method to fix their positions at sea. But recently, a vessel's position can be obtained with high accuracy by using kinematic GPS (KGPS) in addition to a land survey in various fields. We carried out the experiments to compare radar positions with KGPS positions in the KURUSHIMA strait using the training vessel "KOJIMA" with the KGPS receiver on board. The positions of KOJIMA obtained by a radar were accepted from the radar sight which belongs to HONSHU-SHIKOKU Bridge Authority. After KGPS positions were exchanged to the plane coordinates, the difference between the two positions were examined and the results were as follows. 1. The radar positioning accuracy were 26.4m in length and 7.7m in width as 1σ. 2. The errors in width showed the normal distribution. 3. The vessel's course and speed estimated by radar observations often indicated sharp fluctuations. 4. The bearing errors of radar became greater at shorter distance.