An analytical model, simultaneously accounting for gravity, vapor shear along axial direction and surface tension effects on the condensate film layer, is proposed for flow condensation process in horizontal annulus with small/mini diameter inner tube and traditional industrial-used outer adiabatic tube. The analysis shows that, the effect of surface tension can not be neglected for small/mini tube of diameter less than 3 mm o, especially in low vapor quality zone. The shear stress and surface tension influence the flow condensation mainly by means of distributing condensate film uniformly along tube circumference in horizontal annulus. Experiments were conducted to examine the analysis. (C) 2000 Elsevier Science Ltd. All rights reserved.