A two-phase closed thermosyphon (TPCT), a wickless heat pipe, is a heat transfer device that effectively transfers heat through the phase change of the working fluid. Because it shows high thermal performance despite its simple structure, it is used in various thermal management fields, such as heat exchangers for waste heat recovery, solar applications, HVAC systems, and cooling of electronic devices. The thermal performance of TPCT is affected by parameters such as working fluid, operation heat flux, inner diameter, filling ratio, and inclination angle. The effects of parameters such as filling ratio and inclination angle on the thermal performance of TPCT have been studied by many researchers. Several researchers have experimentally investigated the effects of inner diameter on the geyser boiling phenomenon (GBP) and entrainment limit (flooding limit), but the effect of inner diameter on the thermal resistance of the TPCT has not yet been studied. In this study, based on the experimental results of TPCTs with various inner diameters (10~25 mm) and heat flux conditions (2.5 ~ 17.5 kW/m²K), the effect of inner diameter on the thermal resistance of the TPCT was investigated. The result shows that the thermal resistance of the TPCT increased as the inner diameter decreased, regardless of the operating heat flux.