Abstract: By means of visual observation on flow patterns and heat transfer experiments,the flow patterns transitions during flow condensation in small/mini-diameter tubes of 1-5mm,and in turn,the heat transfer mechanisms based on different flow patterns are explored.The results show that,with decreasing tube diameter,the area occupied by annular flow in flow map of inclined tubes increases,that is to say,the flow pattern which is characterized by uniform distribution of condensate film along tube periphery will become dominating.This implies the reducing effect of gravity on flow patterns of two-phase flow in small diameter tubes,the tube wall temperature distribution along periphery becomes more uniform.The corresponding heat transfer experiments also reveal that heat transfer temperature difference has different influences on condensation Nusselt number, Nu ,for different diameter tubes. For example,temperature difference has little effect on Nu for horizontal tube of 1.94mm i.d.,due to the different flow patterns.The present study is helpful to understanding the heat transfer mechanism of flow condensation in small/mini-diameter tubes.