Critical heat flux of a two-phase thermosyphon is usually considered as a kind of phenomenon closely related to “flooding” during a countercurrent flow of vapor and liquid in a tube. Some different criteria to analyze the flooding are first introduced and then what difference exists among them is briefly explained. When criterion of maximum falling liquid rate is applied to the CHF in two-phase thermosyphone, its predicted value is in good agreement with existing CHF data as well as existing CHF correlations. The CHF is constrained by the counter-current flow of vapor and liquid. An improvement of the critical heat flux can be achieved by reducing an effect of the counter-current flow, succeeding in significant enhancement of the CHF by means of a concentric tube.