This paper presents a computer simulation model, PORTSIM, that was developed for the grain terminals of the Port of Thunder Bay. In addition to simulating ship arrival and grain loading characteristics, the model incorporates two distinct features of the port. These features are the movements of ships between berths to load different grades of grain and the impact of the rail-side operation on port performance.The paper first reviews existing simulation models and presents an overview of the operations of the port. Model details are then presented and include its logic and structure, design of experiments, measures of performance, and model input/output. In particular, the model provides several measures of performance, including average waiting time of ships, average queue length, berth utilization, port throughput, and maximum queue length. These measures are useful in assessing future improvements in the port. Some practical applications of the model and areas of future expansion of its capabilities are discussed.Key words: port, grain terminals, simulation model, ship arrival, grain loading, performance measures, practical applications.