Abstract: Based on surface observations from 2002 to 2008, data from a Qingdao buoy, ocean-atmosphere integrated observations, NCEP reanalysis data and satellite data, the climate characteristics and circulation patterns of Yellow Sea fog as well as related hydrologic and meteorological factors are studied. Results show that the number of annual mean fog days has been increasing year by year. Most sea fog, mainly observed from midnight to next morning, happens in spring and summer and moves northward. There are three main synoptic patterns of Yellow Sea fog that are associated with the background of cold fronts, the rear portion of a high pressure or a uniform pressure field. The Yellow Sea fog forms when a warm branch of the Kuroshio encounters with a cold ocean current along the coast, together with some other hydrologic and meteorological conditions like moderate sea temperate, air-sea temperature difference and wind. All those factors provide some basis for forecasting of the formation and development of sea fog.