The Bayan Obo deposit is the largest REE deposit in the world，and is also a giant Nb and Fe deposit，its metallogenic mechanism is very sophisticated．This research has firstly collected phlogopitite from the #3 Nbore body in eastern contact zone（Boluotoushan），and obtained 269.5±3.1 Ma and 249±13 Ma UPb ages for its relict zircon and hydrothermal monazite respectively；then，we have collected the Bayin Obo pegmatite，and obtained 248.9±2.5 Ma UPb age for its hydrothermal zircon．Combining with the reported mineralization ages in the AMH orogenic belt (i.e. Altay—Inner Mongolia—Greater Hinggan Mts.—Lesser Hinggan Mts. orogenic belt)，this research deduces that a strong hydrothermal activity happened at 248 Symbol~A@ 251 Ma in the Bayan Obo area，the time of this hydrothermal activity may represent the mineralization age of the Bayan Obo deposit．On the basis of close relationship between granitic magmatism and hydrothermal ore deposit，we has carried out petrographic，mineralogical，geochemical and geochronological studies on two types of granites（biotite monzogranite and twomica alkalifeldspar granite）occurred in the Bayan Obo area，and determined that the biotite monzogranite（main phase of the Bayan Obo granitic batholith）has syncollisional nature with its emplacement age of 269.8±2.0 Ma，and the twomica alkalifeldspar granite（subsequent phase of the Bayan Obo granitic batholith）has postcollisional nature with its emplacement age of 250.5±6.0 Ma．According to the similarity in age between the twomica alkalifeldspar granite and the Bayan Obo deposit with the observation in the field，this research concludes：① The twomica alkalifeldspar granite is parental rock for the Bayan Obo deposit，its magma came from the deepseated magma chamber in the lower—middle crust；② The magma chamber is the same for the syncollisional granitic magma，meaning that the huge amount of magma in the magma chamber went through ca．20 Ma of fractional crystallization，thus the oreforming materialenriched residual magma occurred at the top of the magma chamber；③ When the tectonic regime converted from the compressional to the extensional states，the residual magma could ascend along extensional fault in the permissive mechanism．A large amount of rare metal（REE and Nb） — halogen（F） — alkali metalenriched siliceous fluid was separated from the residual magma with intensely decreasing pressure and temperature；④ Such siliceous fluid entered the dolomite H8 along the fracture，and REE and Nb ores formed after its metasomatism with carbonates．At the same time，the siderite and ferrous dolomite in the dolomite H8 decomposed，and released Fe2+ and \[CO3\]2-，the former（Fe2+）precipitated as Fe ore after a short migration，and the latter（\[CO3\]2-）combined a small quantity of REE and Nb into metal—carbonate complex，which precipitated as REE and Nb ores in the form of veins in the dolomite H8 or in the rocks outside the dolomite H8．This metallogenic model firstly combines tectonic movement，granitic magmatism and mineralization in the Bayan Obo area，then determines a migration process of energy and material up to the formation of the Bayan Obo deposit；Moreover，this metallogenic model takes account of the profitable factors from the main genetic hypotheses（e.g.，“general or hotwater sediment”，“carbonatite intrusion”and“hydrothermal metasomatism”，etc.）on the metallogenic mechanism of the Bayan Obo deposit；Therefore，it is a comprehensive and distinctive metallogenic model for the Bayan Obo deposit．