An observation was carried out in the winter in 2006 at Pancheng,northern suburb of Nanjing,using tethered balloon system,automatic weather station and other instruments.The boundary layer structures on fog and fogfree days are analyzed and compared.Results show that on the fogfree days the temperature inversions are mainly singlelayer ones,sometimes with short duration doublelayer structures,while the fog mature stage inversions are free from the ground.Compared with the fogfree days,the temperature diurnal range above the average fog top on fog days is larger while the below one is smaller.The timeheight cross section of average wind speed on the fog days is similar to that on the fogfree days.In the stable boundary layer,the wind speed profiles have one or two maximum values.The average wind speed profiles on fog days are similar to those on the fogfree days.No remarkable relationship exists between the temperature and specific humidity profiles on the fogfree days.Differently,these two parameters have a good correspondence between each other in the fog body whereas they vary oppositely during the ascent and descent of the fog top.The stronger temperature inversion,lower temperature and smaller nearsurfacelayer wind speed are background for the winter fog formation in Nanjing.