With increasing demand for medical services, emergency departments (ED) are facing problems such as overcrowding and dissatisfaction. Improving the key performance indicators of EDs has been the focal point of healthcare management. This paper addresses performance analysis of ED of a general hospital. To this aim, a discrete event dynamic modeling approach is used to model the ED processes. The model employs a hierarchical timed Coloured Petri net framework in a concise and detailed way to capture patient flow and care processes within the ED. The simulation model was validated against historical data and then different types of scenarios were used to assess, compare and improve ED key performance indicators, such as patients waiting time,length of stay (LOS), and resource utilization rate. The proposed model helped the hospital policy makers to configure the ED in a way to improve its efficiency and staff satisfaction.