Collocated aircraft observations of microstructure and radiative properties of winter boundary layer clouds over the East China Sea and the Japan Sea have been carried out in January 1999 within the Japanese Cloud and Climate Study (JACCS) program. The first part of the paper describes the in situ measured microphysical and optical properties for two cases of boundary layer winter stratocumulus clouds, which concern, first, a rather uniform, supercooled water cloud contaminated by aerosols and, second, a highly heterogeneous, mixed-phase stratiform cloud. Using the Polar Nephelometer, a new instrument for measuring, in situ, the scattering phase function of cloud droplets and ice particles, the polluted, continental-type stratocumulus cloud can be optically regarded as a liquid water cloud because the measured scattering phase functions fitted very well with those calculated from Mie theory for the directly measured FSSP size distributions. In mixed-phase cloud, the measured scattering phase function shows that ice particles strongly affect optical properties of the cloud, where large number of liquid water droplets with higher extinction coefficient and asymmetry factor values were converted into a much smaller number of large ice crystals with lower extinction coefficient and asymmetry factor. Furthermore, a quasi-stable liquid-topped cloud layer with precipitating ice particles was noticed; the layer may, first, affect the cloud radiative properties and, second, seriously restrict the interpretation of satellite cloud composition retrievals.