In this paper, a systematic approach to assess maneuvering safety of ships in harbors and narrow channels is reported. At first, the sequence of safety assessment is discussed. Combining fast-time simulation and real-time simulation with CGI system, the safety assessment works can be systematically achieved. The deviation from the planed trajectory, which comes from many factors such as wind, ... [Show full abstract] current, can be estimated by use of fast-time simulation that controlling model is fixed. The estimated deviation from the standard can be used as a measure of maneuvering safety, and it will be helpful to set scenarios of real-time simulations. Real-time simulations are carried out for assessing maneuvering safety including human factors. By applying fast-time simulation's control model to real-time simulation's trajectory, and comparing with the both deviation, it is also possible to grasp hidden risk of maneuvering. The examples of safety assessment are finally reported and the effectiveness of the procedure mentioned above is shown.