This study examined saturated water/alumina nanofluid pool boiling on a 10-mm-diameter, upward-facing, plain copper heater under atmospheric pressure. In order to investigate the effect of a nanoparticle coating on the boiling phenomenon near critical heat flux, we compared the boiling characteristics of water boiling before, alumina nanofluid boiling, and water boiling after alumina nanoparticles. The unusual boiling characteristics (bending point in the boiling curve at a high heat flux), CHF enhancement, and gradual increase of wall temperature just after the CHF (instead of the rapid increase of wall temperature) were analyzed based on high-speed visualizations. A thicker macrolayer resulting from the nanoparticle coating is estimated to influence the CHF enhancement, which played a role similar to a porous medium. Thus, we revealed that the nanoparticle coating induced the transition boiling with the nucleate boiling and the partial dry-patch (vapor film in the large vapor mushroom) at a high heat flux as the role of porous medium, which explained the bending point in the boiling curve and the gradual increase of wall temperature. (C) 2013 Elsevier Ltd. All rights reserved.