Abstract: Based on the annular flow pattern, the mathematical model of condenser pipes with small diameters was established. The effect of surface tension on the liquid/vapor interface causing the pressure difference between liquid and vapor phases on the same cross section and the interaction between the liquid and vapor phases including the frictional and momentum-transfer shear stresses were considered in the model. The variations of the liquid and vapor phase pressure drops, the thickness of the liquid film, shear stress on the liquid/vapor interface and so on along the pipe length can be obtained by solving the model. Based on the modeling results, the conclusions below can be drawn: the two-phase pressure drop along the pipe length increases nearly linearly; the average velocity of vapor phase first increases and then decreases, but the variation range is small and far bigger than that of liquid phase; the momentum-transfer shear stress increases as the thickness of the liquid film increases, and can not be ignored compared to the frictional shear stress; the convective heat transfer coefficient at the inner wall of the pipe decreases as the thickness of the liquid film increases, but it is still comparatively bigger when the condensation is almost completed.