In this paper, the authors present liquid film characteristics during trans
ition from countercurrent annular two-phase flow to flooding in a vertical
tube. Time variations of liquid film thickness at four different heights in
the tube are measured. Their power spectra are used to examine interfacial
wave characteristics. The instability of a liquid-gas interface in counter
current annular two-phase flow is studied using linear and nonlinear method
s. In the linear analysis, the Rayleigh equation is applied. In the nonline
ar analysis, the long-wave analysis considering the base harmonic and harmo
nics is performed. The critical wave number obtained from experiments is co
mpared with the stability maps. In the inviscid analysis, most of the criti
cal wave numbers for each flow regime are located in the unstable region ob
tained from the theoretical instability map; therefore, this analysis canno
t explain the transition to flooding. The wave number obtained from the pre
sent experiment decreases with increasing gas flow rate. In the nonlinear a
nalysis, the critical wave number increases, and ultimately, the critical w
ave number from the experiment is found in the unstable region immediately
before flooding. This analysis is found to be consistent with the experimen
tal results.