Instability of the condensate film in a small-diameter-tube condenser was investigated by using an integrodifferential approach. The disturbance wave parameters were predicted based on the characteristic equation derived in this study, and the results were in good agreement with the available experimental data reported in the literature. Capillary blocking taking place in a small-diameter-thermosyphon condenser was also examined. The proposed mechanism for capillary blocking reasonably explains the observed two-phase-flow phenomena during formation of capillary blocking.