An experimental investigation was conducted to study-bubble sliding in subcooled flow boiling of water in a vertical narrow rectangular channel. Bubble behaviors were recorded using a high speed digital camera and an automatic digital image processing algorithm dealing with bubble sliding parameters (including bubble numbers, bubble velocities and bubble equivalent diameters) was proposed. The bubble image sequences were analyzed to obtain the distribution of bubble diameters and bubble velocities as well as their mean spatial-temporal value. Most of the bubbles in the experiments slide along the heated surface, but not lift off the surface, as is usual in a conventional channel. Two types of bubbles with different behaviors were observed in the experiments. The first type has a shorter lifetime and their diameters or volumes change a lot through rapid vaporizing and condensing. On the contrary, bubbles which were classified as the second type have a longer lifetime and their diameters or volumes change slowly. Bubble sliding velocity and bubble equivalent diameter distribution profiles were obtained based on the results of the automated analysis. The distribution of bubble sliding velocities and bubble equivalent diameters indicates that both of them cover a wide range. At last, mean bubble velocities, mean relative velocities between bubbles and local liquids and bubble densities under different working conditions were obtained to manifest the effects of mass flow rate, inlet subcooling and heat flux. Crown Copyright (C) 2012 Published by Elsevier Ltd. All rights reserved.