Annular flow, characterized by liquid film attached to the tube wall while gas phase with entrained droplets flowing continuously through the tube core, widely exists in many industrial processes including transportation of crude oil and natural gas, evaporative cooling, and nuclear reactors, to name a few. The liquid film has been studied by several researchers since it is crucial in mass, momentum, and energy transfers between liquid and gas phases. However, most of the experimental investigations available in the literature are limited to air-water annular flow under the near atmospheric condition while the physical properties of working fluids deviate from the steam−water annular flow under the boiling water reactor (BWR) operating condition (7MPa, 285°C). Hence, it is significant to clarify the behavior of the liquid film under high pressure and temperature. In this work, experiments of gas-liquid two-phase annular flow in a tube with an inner diameter of 5.0 mm were conducted. We use 95% ethanol aqueous solution and HFC134a gas as working fluids whose properties under comparatively low pressure and temperature conditions (0.7 MPa, 40 °C) are similar to those of steam and water under BWR operating conditions (7 MPa, 285 °C). The constant electric current method (CECM), one type of conductance method, is employed to measure the time-varying liquid film thickness. Based on the experimental results, a detailed study of liquid film characteristics in the annular flow of HFC134a gas and ethanol aqueous solution is presented in this work. Characteristics including base, average, and maximum film thickness, and wave velocity are measured and studied. The prediction models available in the literature for average film thickness and wave velocity are comprehensively reviewed and evaluated by the experimental data. A new model is developed for the wave velocity prediction.