Two wind maxima with jet-stream characteristics often occur, one above the other, in the lower troposphere by the south side of the rainstorm area over South China in summer.The lower one lying in the boundary layer above the surface friction is designated as the Southerly Boundary Layer Jet (SBLJ). Its structure and formation are investigat-ed in connection with its overlying counterpart, the Low Level Jet(LLJ). It is found that the general trend of higher pressure to the eastand low to the west in the surface layer provided by the western Pacific Subtropical High and the Indian Monsoon Low gives rise to the large-scale southerly flow while the secondary vertical circulation cellforced by the LLJ and the associated frontal shearline superimposes asub-synoptic southerly current on the general flow. The combination ofthese currents result in the SBLJ. Any fluctuation of the LLJ will be reflected on the activity of the SBLJ.