Located in Liangcheng uplift of Inner Mongolian massif (Axis) within the Northern China craton, the newly discovered Caosiyao deposit in Xinghe county, central Inner Mongolia is a giant molybdenum deposit. During the Mesozoic orogenies, intensive tectonic and igneous activities resulted in large scale granitoid magmatism in the Caosiyao mineralized district. The granitoid porphyry stocks and dyke swarms were emplaced in the Achaean metamorphic rocks of the Jining Group. Rock forming minerals of the ore bearing granitoid porphyry are mainly quartz, microcline, microperthite and plagioclase associated with minor biotite and sericite. Accessory minerals are magnetite, apatite, zircon, sphene and allanite. The zircon samples from the ore bearing granitoid porphyry stock have yielded LA ICP MS U Pb ages ranging from 134 Ma to 131 Ma, indicating the stocks and dykes may be formed during the Early Cretaceous tectonic activity. Molybdenum mineralization occurs in both Achaean metamorphic rocks and granitoid porphyry stocks as veins, veinlets and disseminated blocks. The molybdenum ore consists of pyrite, molybdenite, chalcopyrite, sphalerite, wolframite, K feldspar, quartz, sericite, chlorite and epidote. As the most important economic molybdenum mineral, molybdenite occurs as impregnation, radioflake, thin film, aggregated flakes and veinlet. Re Os isotopic age dating on four molybdenite samples from the main orebody gives model ages of 1319±23Ma, 1312±23Ma, 1297±20Ma and 1286±24Ma, with an average value of 1304±24Ma, which is similar to that of zircon U Pb age of the ore bearing granitoid porphyry. As the Re Os model ages are in agreement with field geological evidences, and the molybdenite has co existing relations with pyrite and chalcopyrite, it has been suggested that the Re Os model ages represent the ore forming time of the Mo mineralization occurring in the Caosiyao district and its neighboring area. Preliminary studies show that the Caosiyao deposit were formed during the Mesozoic Yanshanian extensional tectonic movements that resulted in the intensive igneous activities. It is believed to be a product of the Early Cretaceous intraplate granitoid magmatism within the Liangcheng uplift. The well developed high K and SiO2 granitoid porphyry stocks and dykes might have acted as the “engines” of the ore forming systems. Most of the Mo was derived from partial melting of the Precambrian continental crust. The ore forming fluid was dominated by magmatic water at the early metallogenic stage, and was characterized by mixture with magmatic water as the major source at the main metallogenic stage. The combined geochemical data, field geological observations and petrological evidence indicate that the Caosiyao deposit belongs to the porphyry type Mo deposit. The genetic model and mineral exploration criteria of the Caosiyao deposit can also be used as useful tools during the comprehensive evaluation of concealed Mo deposits in Inner Mongolian massif of the North China craton.