Urban heavy smog is a serious environmental problem. Based on the field observation in Beijing from November 1 to December 20 of 1999, differences between heavy smog and fog are analyzed, and the structures of the atmospheric boundary layer for heavy smog and fog are discussed. Some results are founded. For example, heavy smog and fog can be distinguished from visibility, relative humidity and long-wave radiation equilibrium. The differences of heavy smog and fog in physical and chemic natures induce the boundary layer profile differences in relative moisture, inversion layer, wind speed, vapor, etc.