Accurate measurements of the interfacial wave structure of upward annular two-phase flow in a vertical pipe were performed using a laser focus displacement meter (LFD). The purpose of this study was to clarify the effectiveness of the LFD for obtaining detailed information on the interfacial displacement of a liquid film in annular two-phase flow and to investigate the effect of axial distance from the air-water inlet oil the phenomena. Adiabatic upward annular air-water flow experiments were conducted using a 3 m long, I I mm ID pipe. Measurements of interfacial waves were conducted at 21 axial locations, spaced 110 rum apart in the pipe. The axial distances from the inlet (z) normalized by the pipe diameter (D) varied over z/D = 50-250. Data were collected for predetermined gas and liquid flow conditions and for Reynolds numbers ranging from Re-G = 31,800 to 98,300 for the gas phase and Re-L = 1050 to 9430 for the liquid phase. Using the LFD, we obtained such local properties as the minimum thickness, maximum thickness, and passing frequency of the waves. The maximum film thickness and passing frequency of disturbance waves decreased gradually, with some oscillations, as flow developed. The flow development, i.e., decreasing film thickness and passing frequency, persisted until the end of the pipe, which means that the flow might never reach the fully developed state. The minimum film thickness decreased with flow development and with increasing gas flow rate. These results are discussed, taking into account the buffer layer calculated from Karman's three-layer model. A correlation is proposed between the minimum film thickness obtained in relation to the interfacial shear stress and the Reynolds number of the liquid. (c) 2007 Elsevier Ltd. All rights reserved.