Abstract: An inner tube was inserted respectively into the boiling section and condensing section to form a new type of configuration of a two phase closed ther-mosypon. Visual observation and heat transfer experiments on the new two phase closed thermosyphon showed not only a better stability in operation, but also a higher heat transfer coefficient. A fluid flow physical model was proposed. On the basis of the Soliman condensing heat transfer model, a condensing model with vapor and liquid flowing corurrently in annulus was proposed and the mean deviation between the calculated values and the experimental values was 19.4%.