The frozen start up characteristics of the two phase closed thermosyphon using water and FC 40 as working fluid to prevents dry out by freezing of water at the condenser when below zero operation condition has been tested and studied. The experimental data showed that the thermosyphon using immiscible 2 fluids thermosyphon start up successfully at a -30℃ cooling condition as melted water by inflow 2nd working fluid to the condenser.