Visualization experiments were conducted to study flow boiling bubble behavior in ribbed rectangular channels at atmospheric pressure. A transparent conductive oxide film was placed on the inner surface of the channel to act as the heating source. Four rib configurations with different inclinations were examined, and the results are compared to those in a smooth channel. Heat and mass fluxes of 13.3-28.9 kW/m(2) and 152-333 kg/(m(2)s) were considered, and the bubble behavior was captured using a high-speed digital camera system. Vapor bubble diameters and velocities were obtained from an analysis of the images, and we found that the bubble diameter decreases with increasing mass flux, which is also associated with an increase in the bubble sliding velocity and an inhibition of sliding bubble coalescence. For the rectangular channel, a rib angle of 45 provides the largest transverse component of the bubble sliding velocity as a result of the axial drag force and guiding force of the ribs. We conclude that a 45 degrees-angle rib offers the best characteristics for flow boiling heat transfer. (C) 2014 Elsevier Ltd. All rights reserved.