The fog is a kind of special weather phenomenon and is closely related with structures of the atmospheric boundary layer. Based on the Tianjin boundary gradient observation system, we have analyzed a fog process from November 28 to December 2, 2010, which can be divided into two episodes, fogⅠ and fogⅡ. The results showed that: first the condensation of water was found from 80 to 100 m heights during fog Ⅰ and then vapor was transported downward during fogⅡ. There was obvious inversion temperature before fog Ⅰ and unstable atmosphere before fog Ⅱ.While fog Ⅱ happened, the atmospheric status was tending to be neutral, and there was a strong inversion temperature at the top of fog. Analysis on characteristics of radiation and pollution during fog process shows that， there was a long wave radiation equilibrium, an inhibition effect to the photochemical smog, and an accumulated effect of small particle and NOX during fog.