We investigated the characteristics of frosting and defrosting on a fin according to its surface contact angle under the winter operating conditions of a heat pump. The static contact angles were 2.5 degrees, 75 degrees, and 142 degrees for hydrophilic, bare, and hydrophobic surfaces, respectively. The frost layer exhibited different frost formations for different surface contact angles during the early stages of frost formation. Frost retardation was observed on the hydrophobic fin, but the effect was not significant. The frost layer was thinner and the average frost density was higher on the hydrophilic surface than on the other surface-treated fins. The effect of surface treatment on defrosting time was found to be insignificant. However, the ratio of residual water on the hydrophilic surface was significantly smaller than those on the other surfaces. Because of this, in repeated frosting/defrosting experiments, the fin of the hydrophilic surface was found to show almost the same frosting/defrosting behaviors as those of the first run. (C) 2011 Elsevier Ltd. All rights reserved.