The purpose of this thesis was to grasp the process of emergency department (ED) and to find out the simulation model which produces the most reliable outcome. It is the first time ever to apply system dynamics approach for ED. In this paper, schedule pressure was accounted for the development of ED operating system. Workload of ED staffs is varied depending on the number of ED patient visits which affected by time of days and situation of ED operating efficiency. Therefore the simulation model without accounting this complex dynamic situation may produce an outcome which is different from the real life situation. In this study, we developed the simulation model, which was based on the ED at the Severance hospital in Seoul, Korea. The results from this study were as follows: 1) Wait times for ED patients to receive hospital bed were applied to the ED simulation model so that the best representation of actual ED system can be developed. 2) For the first time, it was found that the schedule pressure can actually affect the ED operating system. As number of arrival patients increases, the number of patients in ED also increases and it leads the increased number of patients who required treatment. This also increases schedule pressure of ED staff and it consequently causes changes in both working hours of ED staff and treatment time per patient in ED. 3) It was found that approximately 40% of Oliva’s function is the most appropriate effect level of schedule pressure to apply for understanding of actual ED operating system. For the purposes of this study, system dynamic was used to conduct the simulation model and analyze the corresponding impacts on the actual ED system. The simulation model in this study, was based on the Emergency Department at the Severance hospital, however the methods used in this study can be applied to analyze ED operating system of other hospitals. The system dynamic approach is essential to develop the model which is an accurate representation of the actual system. Therefore a model based on system dynamic can help hospital, bank, factories, supply chains and other service areas to analyze their operating system. In this study, the stochastic simulation of the ED was used as the methodology for examining the ED system performance and it evaluated the effects of complex dynamics on ED operating efficiency.