The Dexing porphyry copper deposit, Jiangxi Province, is the largest porphyry copper deposit in China, and there have been some controversies on the ore–forming source of this deposit. This study has carried out the Pb isotope analyses of pyrites from the Tongchang and Fujiawu mine. The results show that the two mines have the consistent Pb isotopes, with 206Pb/204Pb ratios ranging from 17.954 to 18.320 (with a mean of 18.097), 207Pb/204Pb ratios ranging from 15.407 to 15.517 (with a mean of 15.476), and 208Pb/204Pb ratios ranging from 37.888 to 38.153 (with a mean of 37.999), respectively. The Pb and previous S isotope features indicate that the ore–forming sources are mainly derived from the upper mantle, other than the Neo–proterozoic meta–sedimentary rocks of Shuangqiaoshan Group. On the basis of comparisons of more than twenty molybdenum–bearing deposits worldwide with porphyry type, skarn type and quartz–vein type, we further attribute the high Re–187Os contents of the molybdenites to a mantle origin. The large scale copper deposit should be emplaced in a continental arc setting coupled with westward subduction of the palaeo–Pacific plate, and formed by the Fe3+–rich fluids/magmas released by the P–MORB interacting with the lithospheric mantle wedge.