This paper studies how to estimate container terminal capacity for long-term port development planning. Traditionally port planners have calculated terminal capacity by UNCTAD`s method in which a queuing theory is applied to a port terminal. However, the investigation of domestic container terminals indicates that its assumption is not valid and there is all the difference between theoretical result and actual results. This study proposes the method using a computer simulation in order to affect the characteristics of a ship scheduling and a process time of a containership in a container terminal. The experimental results show that the proposed method affects much better the relationship between the terminal capacity and its service level than the existing method.