Heat pipes are effective heat exchangers that have a wide range of applications because of their ability to passively transfer large amounts of heat. Research into heat pipe technology has dramatically increased over the last decade and, more recently, has incorporated the use of visualization to help researchers gain a better understanding of the boiling phenomenon and heat transfer occurring inside a heat pipe. Neutron radiography is one method of visualization suitable for use in heat pipe investigations due to unique attenuation characteristics of neutrons attaching to various materials. In this study, an aluminum-based heat pipe was tested using working fluid filling ratios from a 10% to 90% capacity. Visualization using neutron radiography was conducted at a neutron radiography facility, RN1, under the supervision of the Centre of Science and Technology of Advanced Materials (PSTBM), National Nuclear Energy Agency of Indonesia (BATAN). Using temperature and pressure sensors, this study revealed that the optimum value of working fluid filling ratios directly correlates to the pressure inside a heat pipe and the size of vapor space available. The neutron radiography facility maintains high neutron flux at 10⁶–10⁷ n/cm² s; high quality images were captured utilizing this radiography visualization technology. The captured images demonstrate that the boiling phenomenon inside a pressure-reduced heat pipe varies when compared with the boiling phenomenon at atmospheric pressure. The visualization result also shows the importance of wick structure in pumping return condensate from the condenser to the evaporator.