The giant Yulong porphyry Cu (Mo-Au) deposit, located in the eastern Tibetan Plateau, is a typical post-subduction porphyry Cu +/- Mo +/- Au deposit formed in an intracontinental strike-slip fault system, and it is also one of the largest porphyry Cu deposits in China with reserves of 6.5Mt Cu @ 0.62% and 0.41Mt Mo @ 0.042%, respectively. In this study, sulfur and lead isotopic compositions of sulfides and ore-related porphyry samples were determined to constrain the sources of ore-forming materials and ore-forming process. Sulfides (e.g., pyrite, chalcopyrite and molybdenite) from phyllic alteration zone, where the Cu and Mo mineralization mainly occurred, have uniform delta S-34 values ranging from -0.6 to + 2.1 parts per thousand (average + 1.1 parts per thousand), which indicates a magmatic origin of the sulfur. In situ Nano-SIMS sulfur isotope analysis further reveals that pyrites formed in potassic alteration stage have systematically negative delta S-34 values ranging from -0.2 to -9.7 parts per thousand (average - 4.5 parts per thousand), whereas pyrites formed in phyllic alteration stage have systematically positive delta S-34 values ranging from + 1.1 to + 3.2 parts per thousand (average + 2.1 parts per thousand). This regular variation is considered to be resulted from disproportionation of aqueous SO2 and subsequent sulfate reduction, further suggesting a magmatic origin of the sulfur. The sulfides have initial Pb isotopic compositions ((Pb-206/Pb-204)(i) = 18.616-18.812, (Pb-207/Pb-204)(i) = 15.670-15.740, (Pb-208/Pb-204)(i) = 38.956-39.080) deviating from those of the ore-related porphyries ((Pb-206/Pb-204)(i) = 18.549-18.718, (Pb-207/Pb-204)(i) = 15.647-15.662, (Pb-208/Pb-204)(i) = 38.794-39.888), but more close to those of the ore-hosting strata ((Pb-206/Pb-204)(i) = 18.423-18.761, (Pb-207/Pb-204)(i) = 15.654-15.713, (Pb-208/Pb-204)(i) = 38.867-39.062). The shift of Pb isotopic compositions of sulfide minerals toward those of ore- hosting strata indicates that the ore-hosting strata had provided a significant amount of Pb for ore formation, and most probably suggests fluid-rock interaction processes in the hydrothermal system. Our study represents a case study in which ore-hosting strata had provided a significant amount of ore-forming materials for porphyry Cu +/- Mo +/- Au formation in the post-subduction setting.