Nowadays, many of changes have arisen in maritime traffic due to the enlargement of ship’s size and improvement of ship’s speed. It is common that the risks of handling a ship in narrow water area is increasing according to increase traffic volume. In order to correspond to these changes and risks, it should be necessary to make sure the relation between proper width of route in fairway and allowable traffic volume. The conventional method of designing a fairway takes into consideration of movement by one ship of maxi-mum size. But the congestion by traffic volume is not taken in the concept. In a while, a case of road design is generally considered about a traffic volume. From such a view point, this research proposes the method of de-termining fairway-width in consideration of traffic flow. To evaluate traffic congestion in a route, the Envi-ronmental Stress model is adopted as the index of standard, using traffic simulation with avoiding a collision for reproducing traffic flow.