The Bainaimiao copper deposit, Inner Mongolia, is located in early Paleozoic Bainaimiao island-arc belt of the Mandula-Huolinguole arc-basin system in Xing’ An-Mongolia Orogenic Belt. The deposit is comprised of the northern and the southern ore belts. The host rock of the southern ore belt is greenschist, whereas the orebodies in the northern belt mainly occurred in the granodiorite porphyry. There is still controversy over the forming ages of the deposit and Bainaimiao formation for quite a long time. In this paper, two of the SHRIMP U-Pb ages of zircons obtained from the chlororite-plagioschist-the third member of Bainaimiao Group and granodiorite porphyry, are 439.8±6.7Ma and 433.9±2.2Ma，451.8±3.0Ma respectively, and the Re-Os isotopic age of molybdenite is 433.9±3.1Ma. Combining with the isotopic ages obtained by predecessors, it is suggested that the ages of the rock-forming of Bainaimiao formation and granodiorite porphyry ,and the ages of the ore-forming of Bainaimiao copper deposit formed in late Ordovician to eraly Silurian. Both of them were formed in the Caledonian tectonic-magmatic activity. Key words: Chlororite-plagioschist; Granodiorite porphyry; Zircon SHRIMP U-Pb dating; Re-Os isotopic dating; Bainaimiao copper deposit