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