Abstract: The axial heat transfer performance of a natural circulation, which could be used in the cold neutron source(CNS), was experimentally investigated. A two-phase closed thermosyphon system using Freon-113 as work fluid was established. Three kinds of possible flow modes were observed, namely, subcooled boiling oscillation under lower heating flux, stable flow and heat transfer region and dry-out mode under higher heating flux. The result shows that the CNS type coaxial thermosyphon has distinct dynamic performance obviously different from that of a natural circulation in the boiling water (reactor)(BWR) as well as that of conventional single tube thermosyphon.