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The data of hourly surface intensive observation, captive balloon sounding, surface and radiosonde observation and NCEP are used to study the boundary layer structure and its formation causes of a fog event on 30 November and 1December 2004 over Beijing. The esults show that during the fog event, there was a trough crossing Beijing in the atmospheric boundary layer, weak wind and air cooling by radiation near the surface at night. There is a convergence area to pool the moisture air in the atmospheric boundary layer. The fferent structures of temperature, moisture and wind are revealed during the formation and growth of the fog in the atmospheric boundary layer. The formation causes of temperature and moisture features are revealed by dynamic diagnosis. The differences from the typical fogs are also discussed.