Abstract: This contribution analyses the geochemical features of carbo n and oxygen isotope compositions for a REE-rich carbonatite dyke which is loca ted at Dulahala and lies 2 km NNE of the eastern ore body of Bayun Obo giant RE E-Nb-Fe min eral deposit. The dyke cuts across H1 coarse grained quartz sandstone and H3 sha les of the Proterozoic Bayun Obo Group. The analytical results indicate that car bon isotope compositions of the carbonatites vary in a narrow range, with δ 1 3C values between －6.6‰ and －4.7‰ (PDB), which are within the mantle value s ( －5±2‰); while oxygen isotope compositions δ18O have a large range of v ariation from 11.9‰ to 17.7‰ (SMOW) which are apparently higher than mantl e values (5.7±1.0‰), It requires that the carbonatite dyke is more li kely to be related to exchange with low temperature hydrothermal fluids during m agma crystallization or at some later stage after carbonatite magmatism. Both ca rbon and oxygen isotope fractionations between dolomite and calcite from the dyk e is less than 0‰, being in conflict with the sequence of 13　C - and 18　O-enrichment in dolomite relative to calcite at thermodynamic equi librium. The disequilibrium dolomite-calcite paires are not cogenetic, and dolomite might b e of secondary origin, which could be related to the extensive dolomitization in the Bayun Obo area.