The mechanism of countercurrent flow limitation (CCFL) was investigated analytically and quantitatively for the existing experimental results for vertical circular tubes, adopting a criterion that the CCFL condition is given by maximizing falling water mass velocity with respect to void fraction for the whole flow channel. Through the analysis, it was found that significant factors in the CCFL condition were the interfacial friction factor between liquid and gas based on the relative velocity, wall friction factors for laminar, transition and turbulent liquid flows and the effects of tube diameter and length. Analytical results gave very good predictions of the experimental results for vertical circular tubes of 19 to 140mm in diameter and 12.7 to 1520mm in length, under atmospheric pressure.