A comprehensive observation of boundary layer structure was carried out at Pancheng town in the north suburb of Nanjing during fog days between 24th and 27th December,2006,using a DigiCORA Ⅲ system for boundary layer elements sounding and an Envirostation TM for surface element observation.The physical mechanism and boundary layer structures of the advection fog are studied thoroughly,and the results are as follows.The formation and maintenance of the relatively typical advection fog was mainly determined by the warm/wet airflow and the systematical sinking motion,and the invasion of dry/cold air resulted in the dissipation of the fog principally.Besides,the fog two-layer structure appeared during the descending period of fog top.The middle temperature inversion,belonging to the subsidence inversion and advection inversion,was a primary inversion,and its intensity center was near or under the fog top.There were several speed maximum values of wind in the vertical.The jets in the middle layer and in the lower layer exhibited strong and weak alternation after the coalescence of the middle layer jet and the weaker ones in the middle-upper and upper layers.While the speed was low,the wind field approached homogenous,and during the period of dissipation,the wind field of the lower layer showed a linear distribution of wind speed with height.The moisture source of the fog was mainly from the warm/wet air flow.There was a good spatial-temporal distribution correspondence between moisture and wind fields,and the intensity center of the main temperature inversion was also the intensity center of the moisture inversion,so that the spatial-temporal distribution of the moisture was mainly determined by both the wind and temperature fields.