In this study, naphthalene heat pipes were produced and filled with different materials to test the heat transfer performance. The relationship between the pipes' diameters, liquid content, wall temperature distribution, and transmission power was studied. An explosion test was designed to assess the operating temperature limit of the naphthalene heat pipes. The results demonstrated an optimum filling amount to optimize the pipes' heat transfer performance. In practical applications, it is very important to strictly control the wall temperature of naphthalene heat pipes. When the wall temperature of the evaporation section reaches approximately 600°C, they can easily explode.