Abstract: Carbonate rocks from Bayan Obo, Inner Mongolia, can be divided into sedimentary limestone and dolostone (H8a), coarse-grained dolomite marble(H8c) fine-grained dolomite marble(H8f)and carbonatite dykes(D)based on their textures and occurrences. H8, which shows very weak metamorphism and deformation is distributed to the north of the Kuangou fault zone, occurring together with quartz sandstone, sandstone and shale to form the Bayan Obo Group of middle Proterzoic age of normal sedimentary origin. In contrast to H8a, however, H8c and H8f are distributed to the south of the Kuangou fault zone and have distinctive petrologicl features, both of which are strongly deformed and sheared and metamorphosed to dolomite marble. H8 which hosts ores is mainly consisted of dolomite, apatite, magnetite, and pyroclore etc. These minerals show strong lineation which is the result of numerous clusters of euhedral magnetite and apatite drawn out into granular schlieren. The lineation resulted from shearing stretching and deformation is interpreted to be related to the regional thrusting and folding H8, is close to the ores and is composed of dolomite, magnetite, monazite, bastnaesite and parisite etc. The fine-grained monazites are lined to fil1 fractures in dolomites. Similar to H8c, the minerals in H8f also show strong lineation. Electronic microprobe is used to analyze the chemica1 compositions of carbonate minerals from these rocks at Bayan Obo. Results show that the main rock-forming minerals-dolomite or ankerite in ore-hosted rock from giant Bayan Obo REE-Nb-Fe ore deposit are similar to those from carbonatite dykes(D), which occur 0 to 3.5 km northeast to the Bayan Obo deposit, and cut the Archean granitic gneiss and migmatite of the Wutai Group and the Proterozoic detrita1 rocks of the Bayan Obo Group. A1l these minerals contain MnO higher than 0.50 wt％ and SrO greater than 0.15wt, which are identical to the typical signatures of the mantle derived carbonatite elsewhere in the world. However, they differ completely in chemica1 compositions from calcite and dolomite, with MnO and SrO content 1ess than 0.1 wt％, in typical sedimentary 1imestone and dolostone (H8a) north to the Kuangou fault. The variation of MnO and SrO content in dolomite from ore-hosted dolomite can be interpreted by fractional crystallization of a carhonatltic magma origining from the mantle, which results in REE enrichment in the evolved residual magma. MnO and SrO content in carbonate minerals can be used as an important typomorphic characteristics to identify the genesis of minerals. It is suggested that the value of MnO and SrO content in carbonate minerals higher than 0.15 wt％ is an indicator of crbonatite from sedimentary carbonate rocks.