‘Yamase’ clouds are typical maritime boundary-layer clouds, for which present state-of-the-art weather prediction models and general circulation models are currently unable to simulate adequately. Yamase clouds frequently appear over the western North-Pacific region, off the east coast of the Sanriku district, Japan. They occur during the summer season under cool easterly winds blown from Okhotsk anticyclones. Here we discuss optical and microphysical properties estimated from shipboard experiments and satellite remote sensing for Yamase clouds observed in June 2003. This was the first time we were able to observe time-varying features of the marine boundary-layer accompanied with Yamase clouds from formation to decay. At the ship site, the cloud-base height and liquid-water-path, time-averaged during the June Yamase event, were 270 m and 60 gm^-2, respectively. At the same time it is revealed, from satellite remote sensing using the contemporary AVHRR/3 data on morning orbital NOAA-17, that the Yamase clouds were rather thin, stratiform low-level clouds, and their physical parameters gradually grew with time during the Yamase period. The optical thickness, effective particle radius, and liquid- water-path, averaged over the time and area, were estimated to be about 12, 12 µm, and 110 gm^-2, respectively.