Lean thinking is one of the most modern and well-established methodologies which provide an effective structure to improve the performance of production and service processes including healthcare systems. The objective of this study is to minimize the length of stay (LOS) of patients at the Emergency Department (ED) by using the principles of lean production, and thus to increase the satisfaction of patients and medical staff. Process improvement methodology and tools were applied in parallel with lean principles to detect the critical quality factors affecting LOS, and to seek non-value added activities in the emergency service processes. Moreover, other tools of Lean-Kaizen such as fishbone diagram, brainstorming, and VSM are used to analyze the root causes of waste and propose the improvements. Appropriate improvements were generated based on lean philosophy coupled with a simulation model.