The evaluation method of own ship's action by the distance of closest point of approach (DCPA) and the time to CPA (TCPA) is common method. But this method is attended with an inevitable fault like as two-dimensional function (distance and time) and difficulty in considering estimated error of targets vector. To solve these faults, new evaluation value of action for collision avoidance (EVAC) was considered. The underlying principles of EVAC is to evaluate own ship's action by estimated targets distance within a fixed time. The results of investigations about this method are summarized, (1) The physical meaning of this value is clear. (2) It is easy to take into consideration an estimated error of targets vector. (3) It is expected to be used a decision problem for collision avoidance in a congested water.
