The role of the disturbance waves for transferring liquid toward the top of a horizontal tube wall to cope with the drainage due to gravity is investigated by the direct numerical simulation (DNS) which is based on the continuity equation and the Navier-Stokes equations in three-dimensional cylindrical coordinate system. The level set method is used for capturing the interface between gas-liquid two fluids. Developing flow from a separated to an annular flow is simulated by using this numerical technique, and the liquid film formation is reproduced. It is demonstrated by this calculation result that liquid is transferred in the circumferential direction as the liquid film by the pumping action of disturbance waves which has been proposed by one of the present authors. That is, the pressure gradient formed within a disturbance wave in the circumferential direction plays an important role for the liquid film formation in a horizontal annular flow. (C) 2003 Elsevier Ltd. All rights reserved.