The authors have already proposed the new mathematical model of ships' maneuvering motions at low speeds. Combining the mathematical model with results of captive model tests in shallow-water conditions, ships' maneuvering motions in harbours are simulated at ease. In the present paper, an attempt is done to make a safety assessment of harbour maneuvers of some VLCC, utilizing the above simulation technique. To assess the safety in the harbour maneuvers, it is necessary to make plans of approaching to the berth and departing from it by skillful pilots, being based on the informations on the harbour, traffic rules, environmental conditions, ship's specifications and so on. Next, to verify the plans, basic maneuvering abilities in course keeping, course changing, decelerating, turning and so on are checked by the numerical simulations. These results are very useful for the improvement of the maneuvering plans. Real-time simulations of approaching and departing maneuvers conducted by skillful pilots are also beneficial for the total judgement of the safety. After investigating systematically on the above three steps, the final conclusions are achieved, taking the experts' opinions into consideration. It is concluded that the above procedure is very useful for designing ships and harbours and for improving the methods to handle ships in harbours as well as for making safety assessments.