In this paper, the Re- Os isotopic dating of seven molybdenite samples from the Jiazishan Mo (Ag) deposit, Inner Mongolia shows model ages from 143. 1±2. 1 Ma to 147. 2±2. 4 Ma, averaging 145. 8±1. 4 Ma (MSWD=1. 5), and a good isochronic age of 147. 4±1. 4 Ma (MSWD=1. 5). It indicates that the Jiazishan Mo (Ag) deposit was formed in the late Jurassic which belongs to the large- scale Mo mineralization of the Yanshan period in eastern China. The Re content of the Jiazishan Mo (Ag) deposit is between 37. 62×10-6 and 401. 34×10-6, averaging 121. 28×10-6. As Re content in molybdenite can be used as an important indicator on source of ore- forming material, it is likely that the ore- forming material in the Jiazishan Mo (Ag) deposit was derived from the crust- mantle mixed source.