The multi-purpose gamma irradiator in Indonesia, with Co-60 radioactive source, is potentially resulting decay heat generation when the accident occurred. The vertical straight wickless-heat pipe is chosen as an alternative technology to remove the heat generated when the accident occurs, such as prolonged station blackout, so the safety aspect of gamma irradiator enhance. The objectives of this research were to investigate the thermal performance and heat transfer phenomena of vertical straight wickless-heat pipe model as a gamma irradiator passive cooling system. The working fluid filling ratio, hot water temperature source, and air-cooling velocity are considered as factors that influence the vertical straight wickless-heat pipe thermal performance. The heat pipe with a total length of 1000 mm was operated with the variation of working fluid filling ratios of 55, 65, 75, 85%, hot water source temperature of 60, 70, 80, 90°C, and cooling air velocity of 1, 1.5, 2.1, 2.8 m/s. The lowest thermal resistance of 0.03 °C/W is obtained from the experiment with filling ratio of 55%, air coolant velocity of 1 m/s, and hot water temperature of 90°C. The experiment investigation results show that the vertical straight wickless-heat pipe had a higher thermal performance to remove the decay heat generation. Heat pipe could be chosen as one of passive cooling system alternative technology to enhance the safety factor of gamma irradiator when an accident occurs.