An experiment has been carried out to elucidate the critical heat flux (CHF) of an open thermosyphon with a bottom heated chamber and to enhance the CHF using a concentric-tube connecting between the heated chamber and an upper cooling chamber. The CHF data are measured for the saturated liquid of R113 at a different pressure and different dimension of concentric tubes. The CHF data without the inner tube an in good agreement with the existing correlation and analytical result. Heat supplied to the bottom chamber is absorbed by evaporation of liquid and vapor generated thereby makes a counter-current flow of vapor and liquid in an ordinary thermosyphon without the concentric-tube. On the other hand, in the thermosyphon with the concentric-tube by which the counter-current flow can be controlled well, the CHF can be improved as much as several times of the CHF without the inner tube at an optimum condition. The CHF is increased with an increase in the diameter of inner tube up to its optimum diameter and then decreases continuously as the inner tube diameter approaches the outer tube diameter.