Nowadays, hospitals are facing overcrowding of patients in outpatient treatment. It caused many problems because the patient takes a long time to wait. The long waiting times would lead to patient dissatisfaction. This study aims to improve the performance of the outpatient queuing system. The simulation technique is applied to analyze the existing system. First, the system description is constructed to determine the outpatient process flow. Next, the required data is collected, and the probability distribution is then fitted. The simulation model is developed using ARENA software. The model is then verified using a run check model and validated using paired T-Test. The simulation reports are analyzed, and it obtained the problems of long waiting times of internal medicine patients and the high utilization of internal medicine Doctors. Scenarios are then suggested to improve system performance. Scenario-3 of addition one internal medicine Doctor and develop a booking system is selected as the best scenario. The scenario can reduce the average waiting time of internal medicine patients from 97.4 minutes to 8.5 minutes and internal medicine doctor utilization from 87% to 75%. This research hopes can help hospital managers continuously improve their service quality and patient satisfaction.