Experimental study on the closed two-phase thermosyphon loop system using the F-113 were performed. The results showed that the heat transfer efficiency was raising in accordance with increasing of the temperature difference between heat source and heat sink. And it showed that the pertinent fluid charge also existed. It can be also found that the relation between the loop pressure and the heater exit temperature is available to find the datum point which can decide the performance of a system. (author). refs., figs., tabs.