The effects of h/s (tip gap height-to-span ratio) on heat/mass transfer on the bottom surface of a full coverage (FC) winglet, which is installed on a plane (flat) tip, have been investigated by employing the naphthalene sublimation technique along with oil film flow visualizations. In this study, the tip gap is widely changed to be h/s = 0.68%, 1.02%, 1.36%, 1.70% and 2.04% for a winglet width-to-pitch ratio of w/p = 10.55%. From the present measurements, several high heat/mass transfer rate regions are identified on the winglet bottom surface, and various flow phenomena in these regions are discussed in detail. For h/s = 0.68%, heat/mass transfer rate is relatively high even outside the high heat/mass transfer regions. On the contrary, for h/s = 2.04%, there exist relatively low heat/mass transfer rate outside the high heat/mass transfer regions and in particular, very low heat/mass transfer rate is observed on the downstream half of the pressure-side winglet bottom surface. As hls increases, heat/mass transfer rate averaged on the winglet bottom surface decreases consistently in almost linear proportion to h/s. (C) 2015 Elsevier Ltd. All rights reserved.