Heat transfer characteristics from sintered microporous coating surfaces are experimentally investigated for subcooled flow boiling in the horizontal, rectangular small channel with the low aspect ratio. The channel has a hydraulic diameter of 1.98 mm and a heated length of 150 mm. Experiments are conducted at atmospheric pressure with fully-degassed deionized water as the working fluid. The liquid mass flux ranges from 93.6 to 187.1 kg/m 2 s, and the inlet subcooling ranges from 30 to 70 K. The heating surface of the channel is covered with the copper microporous coatings to enhance the nucleate boiling heat transfer. Four coatings sintered with differently sized particles are tested to reveal influences of microporous parameters and identify the optimum structure. A smooth (highly polished) surface is also tested as reference. Although few studies have been performed to quantify the heat transfer enhancement contributed by combining internal porous coatings and small channels, this combination has promising applications in many areas such as air conditioning, chip cooling, refrigeration systems, and many others involving compact heat exchangers.