Based on Pb isotopic data of volcanic rocks from the middle part of Sanjiang ( Nujiang, Lancangjiang and Jinshajiang) and adjacent areas, the authors have determined the Pb isotopic geochemical boundary between Yangtze and Gondwana continental blocks in the middle part of Sanjiang region, southwestern China. The geochemical boundary has the values of 206Pb/204Pb = 18.583, 208Pb/204Pb = 38.575 and V1 = 60, V2 = 52. Compared with standard geochemical boundary values, Yangtze and Gangwana blocks have lower and higher values, respectively. The Pb isotopic geochemical boundary separates Yangtze block in the east from Gangwana block in the west, it corresponds to Luchun-Hongponiuchang belt geographically, to Deqin-Ziga fault tectonically, to the boundary between Jinshajiang and Lancangjiang in terms of tectonic-magmatic belt, and to the Yangla-Luchun-Hongponiuchang polymetallic ore-forming belt metallogenetically.