The aim of this paper is to investigate and analyze the decision makings of maritime officers under the situations that they should control their ships to avoid ship collision. Five factors influencing maritime officers’ judgement were analyzed such as officer’s experience, radar display types, relative ship speed, relative ship position, and ship crossing type. There were no significant differences according to officer’s experience and radar display types. Head-up display on the ship radar was not more useful than north-up display. On the other hand, there were significant differences according to relative ship speed, relative ship position, and ship crossing type. These differences might be due to the biased judgments by the frame effects.