An experimental work was conducted in a reduced-scale downcomer annulus of a nuclear reactor pressure vessel in order to investigate the behavior of liquid films under emergency core coolant (ECC) bypass conditions and to obtain high definition data for validating two-phase flow prediction capability of computational fluid dynamics (CFD) codes. Air and water at ambient pressure and temperature were used as a working fluid, creating a multidimensional two-phase flow in the test section. The main instrumentation was an electrical conductance sensor that was developed to measure the local thickness of the liquid film. The sensor has an array of 24 x 24 measuring points, each of which has 12 mm x 6 mm of the sensing area. The fabrication of the electrodes on a flexible printed circuit board enabled the sensor to be installed on the curved surface. The time-averaged liquid film thickness, which was measured by the developed sensor, shows the influence of the lateral air flow on the liquid film flow, and it was compared with visual observations. As the air velocity increased, a droplet appeared that was created in the thick part of the liquid film, and the thinned liquid film after entrainment could be confirmed from the measured thickness distribution. In this study, qualitative and quantitative analyses of the measurement results show the reliability of the developed sensor and help to understand the liquid film behavior in the ECC bypass phenomenon. Furthermore, the measured thickness distribution can contribute to validating the CFD codes, which have not been validated sufficiently because of a lack of local measurement data for the ECC bypass condition. (C) 2019 Elsevier Ltd. All rights reserved.