The Kelvin Helmholtz instability of phase change interface during filmwise flow condensation in a vertical small diameter tube is studied by means of energy consideration. Based on the rules that the interfacial waving comes from the balance of various forces doing work on the two phase flow system

the mechanism for the effect of the correlating factors on the stability of condensate film is to be explored in detail. The results indicate that

with decreasing tube diameter

gravity and vapor quality

Bernoulli effect and surface tension on vapor liquid interface may exert more obvious influence on condensate film waving

and hence

increase the instability of the film. Condensation and surface tension on the tube wall would stabilize the film

while the film becomes instable only in a certain range of disturbing wavelength. The results of this paper are helpful in understanding the film instability and the transition of flow pattern during flow condensation in a small diameter tube.