It is necessary for calculating the critical range for collision avoidance to set the values of ship's size, speed and action change. For estimating these values with large ships, 122 cases of ship's collision accident were investigated throughly. Then the critical range for collision avoidance for large ship were obtained. And this critical range could cover 92% of collision accidents. Next, the effect of course keeping ability of ship on the critical range was investigated. From the comparison of the simulated critical range where ship's course was switched from starboard to port side, between stable ship and unstable ship, it was found that the difficulty of collision avoidance was increased by the instability of ship's course keeping ability.
