Sea-ice melting processes are inferred from sea-ice and ocean condition data obtained by research cruises of the Canadian Coast Guard Ship Louis S St-Laurent and Germany Research Vessel Polarstern in the Arctic in summer 2007. The relationship between ice concentration observed by the onboard ice watch and the surface mixed-layer temperature measured by CTD and XCTD had a negative correlation. This implies that as ice concentration decreases, the upper ocean temperature increases due to the large absorption of solar radiation. As a result, sea ice melts mostly by warming of the surface water in the less ice-covered region. However, heating of surface water through ponded ice is significant in the highly ice-covered region during melting season. This suggests that transmitted heat through the ice is also important for sea-ice melting especially in the high ice-covered region. Relationship between ice concentration and surface temperature can be used for estimating the bulk heat transfer coefficient between ice and ocean (Kb) and transmitted heat through the ice by fitting the observation. Kb is proportional to the square of surface wind speed. Transmitted heat through thin ice (h = 0.7 m) is estimated to be twice as large as that through thick ice (h = 1.2 m). A simplified ice–upper-ocean coupled model was applied to examine the effect of heat input from open water, thick ice and thin ice. The relations among ice concentration and surface mixed-layer temperature derived from model are consistent with the observed relations. The results of including the effect of transmitted heat through ice as well as open water suggest that transmitted heat through thin ice amplified the open-water albedo feedback mechanism in the high ice-covered region. Recently, the amount of old and thick ice in the Arctic Ocean decreased drastically. As a result, the heat input to the upper ocean through the ice is enhanced and the ice melt is accelerated.