An experimental study of subcooled flow boiling in a high-aspect-ratio, one-sided heating rectangular mini-gap channel was conducted using deionized water. The local heat transfer coefficient, onset of nucleate boiling (ONB), and flow pattern of subcooled boiling were investigated. The influence of heat flux and mass flux were studied with the aid of a high-speed camera. The bubbles were generated more quickly at higher heat flux and the diameters of departing bubbles decreased with increased flow mass flux. Partial dry-out and rewetting process caused by elongated bubble was also observed at lower mass flux. With the increase of heat flux, the surface near exit started boiling firstly, with more sharply increased heat transfer coefficient compared to the surface near entrance. The experimental heat transfer coefficients were analyzed using four existing correlations. The four correlations are all in an error band of -20% to +25%, and Chen correlation tends to have better performance at higher heat transfer coefficient. (C) 2017 Elsevier Ltd. All rights reserved.