The Xiaerchulu gold deposit of Inner Mongolia located in the Ondor Sum Caledonian accretion zone in Xing’an-Mongolia orogenic belt. The present paper is the first time to report accurate Re-Os isotopic data acquired from 5 crystals of molybdenite separated from this deposit. The Re-Os isotopic model age ranges from (260.1±3.6) Ma to (263.7±3.7) Ma and weighted mean is (261.5±1.6) Ma. The Re-Os isotopic analysis also gives an isochronal age of (263.8±4.4) Ma and the MSWD is 1.3. Combined with the geological features of the deposit, the metallogenic time is Early Permian Epoch. The genesis is related closely to Hercynian tectonic - magmatic activity.