In order to investigate the role of sea-ice and leads in the development of marine boundary layer and clouds, we conducted aircraft observations over the ice-covered Sea of Okhotsk during cold-air outbreaks. The relationship between the ice concentration obtained by analyzing visible images of a video camera and the turbulent heat fluxes measured by the aircraft indicates that the heat fluxes over leads were two orders of magnitude greater than those over sea-ice. The difference of temperature between air and sea surface decreased rapidly with increasing the integrated fetches of open water up to 35 km. Beyond 35 km of the integrated fetch of open water, the deepening of boundary layer was significant due to the formation of lead-induced clouds. These facts mean that the accumulated width of leads in the downstream side is an essential parameter for the air mass transformation. The effect of leads in the upstream sea-ice region on the surface heat budget in the marginal ice zone is discussed.