[en] Counter-current annular two-phase flow occurs in wet wall towers and in up-draft condensers. The critical velocity of flow, which makes the flow stable, is called flooding velocity. In annular two-phase flow, the state of liquid film is stable just before flooding effect occurs. As soon as the conditions of flooding occurrence are attained, the amplitude of wave surface of the liquid film increases and the waves on opposite sides are bridged. Because of the bridge between waves, the liquid film is forced into the reverse direction by opposite flowing gas so that the supply of liquid to heating surface stops, and dry out phenomenon occurs. This paper reports on the formula of treating critical velocity based on the instability of liquid film. Also the treatment of critical heat flux in boiling thermosyphon was attempted, and the traditional studies on the flooding velocity in wet wall towers were investigated. By using the formula, the data on the critical heat flux in boiling thermosyphon were investigated and the result was rather satisfactory. (Tai, I.)