The correlating factors for flow condensation in small/mini-diameter tubes are analyzed by energy consideration. Based on the in-situ interface configuration, an analytical model is proposed for predicting the heat transfer characteristics. Both analytical and experimental results show that gravity has a decreasing effect on flow condensation in small/mini tubes. The inclination angle affects condensation heat transfer mainly by distributing gravity in stratifying the fluids and thinning the liquid film. In larger tubes, the former may become the main reason for heat transfer enhancement, especially in high vapor quality zone and at low inlet vapor Reynolds number. Comparison with the experimental data indicates that the proposed analytical model has a good capability for predicting flow condensation heat transfer in small/mini-diameter tubes. (C) 2000 Elsevier Science Ltd. All rights reserved.