Abstract：The Yajishan Mo (Cu) deposit is located in southern Xilamulun Mo (Cu) metallogenic belt on the northern margin of North China block, which is one of the largest Mo metallogenic belts in China. Its major ore bodies occur in the vein like or cataclastic form in the monzonitic granite, strictly controlled by EW- and NE-striking faults. The deposit is obviously of the quartz vein type, and the sulfides in ore veins are dominated by molybdenite, pyrite and chalcopyrite and show veined and disseminated forms in the monzonitic granite. The authors carried out molybdenite Re-Os dating to better understand the mineralization time. Re-Os dating of seven different types of molybdenite separates from the major Mo mineralized bodies gave a weighted average model age of 241.1±1.4 Ma, and six samples of the same type yielded isochron age of (240±3.8) Ma. The Re-Os isochron age represents the ore-forming time of the Mo (Cu) deposit and constrains the age of the monzonitic granite to Early-Middle?Triassic. This result not only provides new evidence for the study of molybdenum deposits in the Xilamulun Mo (Cu) metallogenic belt but also has important significance for discussing the large-scale mineralization in Early-Middle Triassic on the northern margin of North China block. The corresponding geodynamic background was the closure of the Paleo-Asian Ocean.