In this study, the heat transfer characteristics of a two-phase closed thermosyphon were investigated. For the test, a two-phase closed thermosyphon (copper container, FC-72 (C6F14) working fluid) was fabricated with a reservoir which could change the fill charge ratio. The experiments were performed in the range of 50-600 W heat flow rate and 10-70% fill charge ratio. Some findings are as follows. The heat transfer coefficients of the evaporator to the fill charge ratio were nearly negligible. These presented about 1-5 kW/m(2) K with the increase of heat flux and compared with those of smooth surface, showed some enhancement by the grooved surface. However at the condenser, the heat transfer coefficients showed some enhancement with the increase of fill charge ratio by the expanded working fluid pool. And the heat transport limitations appeared in different ways to the fill charge ratio. For the relatively small fill charge ratio (psi < 20%), it presented about 100 W (psi: 10%) by the dry-out limitation. For the large fill charge ratio, it occurred by the flooding limitation and the maximum heat flow rate was about 500550 W (Bo: 26-28), 230 W (Bo: 18.3) respectively and the Kutateladze number was about 1.9-2.1.