The purpose of this research is to find the reasons of overcrowding in the emergency department of an hospital, then to shorten the total stay time of patients. The fact that main causes of the overcrowding exist in the process of the emergency department was discovered by analysis of the data. In order to improve these process, simulation model was developed by ARENA 7.0. Staff's service time, staff's organization, process ratio, and patient's waiting time were estimated in the simulation model in consideration of the decision of the patient's course of action. Several scenarios such as the simplification of the process, the setup of dedicated pathology lab, and mixed method were suggested and evaluated. Total stay time of the patients would be reduced up to 28.45%.