Heat transfer in micro scale two-phase flow attracts large attention since it can achieve large heat transfer area per unit volume. At large flow rate and high quality, annular flow becomes one of the major flow regimes in micro two-phase flow. Heat is transferred by evaporation or condensation of the liquid film, which are the dominant mechanisms of micro scale heat transfer. Therefore, liquid film thickness is one of the most important parameters in modeling the heat transfer phenomena. In the present study, time averaged annular liquid film thickness is measured by laser confocal displacement meter (LCDM), and the gas-liquid interface profile is observed by a high-speed camera. Glass tubes with inner diameters of D = 0.3, 0.5 and 1.0 mm are used. Degassed water and air are used for working fluids, and the total mass flux is varied from G = 100 to 500 kg/m(2) s. Flow patterns are observed and flow pattern map based on Reynolds numbers of gas and liquid flows is suggested. Pressure drop is measured and compared with the prediction using Lockhart and Martinelli parameter. Pressure drop is well predicted with Lockhart-Martinelli correlation. Dimensionless mean film thickness is then plotted against quality, and compared with the annular film model assuming flat gas-liquid interface. Flat interface model overestimated the experimental data. It is considered that the shear stress on the gas-liquid interface in the real annular flow is larger than that estimated in the ideal flat interface model. Prediction using new empirical correlation considering the effect of the non-flat gas-liquid interface showed good agreement with the experiment data. (C) 2015 Elsevier Ltd. All rights reserved.