In the previous study, a double tube structure was employed in the adiabatic tube, which can separate the path of liquid and gas. In the double tube thermosyphon, evaporated gas rise through an inner tube and the condensed liquid flow down through an outer tube. The experimental results indicated that the double tube thermosyphon had higher heat transfer performance than that of the single tube thermosyphon. However, the flooding phenomenon was occurred even in the double tube thermosyphon when the input heat became larger. In this study, a mechanism of the flooding phenomenon in the double tube thermosyphon is discussed.