The Kelvin-Helmholtz instability of phase-change interface during flow film condensation in vertical mini-diameter tube was studied here by means of energy analysis. According to the interfacial boundary conditions, the film thinning effect and the phase-change area enlarging effect by interfacial waves on heat transfer enhancement were analyzed for flow condensation in tubes with different diameter. It is indicated that, in mini-diameter tube, more obvious heat transfer enhancement due to interfacial waves can be expected than that in normal-sized tube, and the interfacial waves enhance the heat transfer mainly by film thinning effect. (C) 2003 Elsevier Science Ltd. All rights reserved.