The Mengku large-size iron deposit occurs in metavolcanic-sedimentary sequence of the Upper Silurian-Middle Devonian Kangbutiebao Formation in the Maizi Basin of the South Altay, with the host rocks being garnet skarn, granulitite, leucogranulitite and marble. Ore bodies are largely concordant with the bedding of the strata, and the distribution of the ore bodies is spatially related to skarn. Precise Re-Os dating of molybaenite from the pyrite- molybdenite-quartz large vein in the No. 1 orebody of the Mengku iron deposit has been carried out in this study, and the results show that the Re-Os model ages of seven samples rang from (261.6±3.7) Ma to (264.0±3.6) Ma, weighted mean ages of (262.9±1.4) Ma with a Re-Os isochron age of (261 ± 6.9) Ma. Re-Os dating of molybaenite suggest that the hydrothermal activity and Cu-Mo mineralization in the Mengku deposit occurred in the late Middle Permian. The ages of 261 ±6.9 Ma of molybdenite provide the latest time of mineralization ages and the iron main mineralization perhaps took place in the Early Devonian. The rhenium contents of molybdenite samples range from (22.74-35.69) μg/g, which indicate that Mo-Cu was derived from both mantle and crust.