Abstract: The National Hospital of Sri Lanka is a major healthcare service provider in the country. The hospital has been expanded with many healthcare facilities. The out-patient department (OPD) is the main division of the hospital which provides their service to over two thousand patients daily. Queuing is a major challenge for healthcare services all over the world, particularly in the developing countries. This study investigates the application of queuing theory to reduce the patients’ waiting time at the OPD of the National Hospital of Sri Lanka. In this study, consultation and pharmacy at the OPD were only considered. The secondary data were collected under the patients’ arrivals and doctors’ roster. The primary data were collected through direct observations during seven weekdays. Data were analyzed and used to model channels for each section of the OPD.