This study experimentally investigated the thermal performance of a two-phase closed-loop thermosyphon with a thermal resistance model for electronic cooling. The evaporator, rising tube, condenser, and falling tube, which are the four main devices, formed a closed-loop system with water as the working fluid. The experimental parameters were the evaporator surface type, fill ratio of working fluid, and input heating power. The results indicated that the evaporator and condenser thermal resistance decrease with increasing input heating power. The condenser thermal resistance clearly increased with increasing fill ratio. A groove-type evaporator surface with 0.2 mm height and 1 mm width had the best performance, decreasing the evaporator thermal resistance about 15.5% compared to a smooth surface. Correlations for evaporator and condenser thermal resistance were also developed, and their precisions, when compared with the experimental data, were about 9.6 and 11.6%, respectively. Because of the intermittent boiling mechanism at 47% fill ratio with input heating power from 60 to 80 W, the temperature showed obvious oscillations with the smooth evaporator surface.