A series of GPS radiosonde and oceanic observations was conducted over the Okhotsk Sea for the first time in July 1998. Under the prevalent anticyclone over the Okhotsk Sea during the observations, distinct differences in the atmospheric and oceanic boundary-layer structure were observed between the fog and non-fog periods. During the fog period, the observed strong surface winds and upward sensible heat fluxes promoted vertical mixing in the atmosphere and ocean. The height of the well-mixed marine boundary layer varied from 200 to 1000m in the atmosphere, and 10 to 15 m in the ocean, respectively. The fog formation occurred when surface air temperature (SAT) was cooler than the underlying sea surface temperature (SST). By trajectory analyses for the observed well-mixed atmospheric boundary layer during the fog period, the boundary-layer air mass that has passed over warm (cool) SST tended to form stratus-like (stratocumulus-like) fog. In the non-fog period, by contrast, atmospheric and oceanic boundary layers were stably stratified due to weak surface winds and downward sensible heat fluxes. Regardless of the air mass trajectory, SAT is warmer than underlying SST during the no-fog period.