Babu ultramafic-mafic rocks, located in the southeastern Yunnan Province, provide researchers a good opportunity to investigate the tectonic evolution of the branch of Paleotethys Ocean. The Yangwan copper deposit, which is hosted by basalts and breccia in Babu area, is considered as a good metallogenic prospective zone. However, the petrogenesis of the Babu ultramafic-mafic rocks and the age of Yangwan copper deposit are still controversial. In order to get the age of ore deposit, the authors applied Re-Os isotope dating to sulfides from the Yangwan deposit. Chalcopyrite and pyrite were picked out for Re-Os isotope analysis. Re-Os isotope dating yielded an isochron age of 269±3 Ma, with an initial ¹⁸⁷Os/¹⁸⁸Os ratio of 0.31±0.17. The well-defined Re-Os isochron, excluding the possible mixed isochron, represents the depositional age of sulfides, which means they were formed at the same time as ultramafic-mafic rocks (~270 Ma). The high initial ¹⁸⁷Os/¹⁸⁸Os ratio was interpreted as entrainment of seawater into pristine hydrothermal fluid. The characteristics of ore and the age of sulfides indicate that the Yangwan copper deposit is a VMS deposit. Though a comparative research on geochemistry between wall rock basalts and other rocks, it is held that all the data support the argument that the Babu ultramafic-mafic rocks are normal middle ocean ridge basalt (N-MORB)-type ophiolites at present. They probably represent the relics of the branch of Paleotethys Ocean.