Ancylte in carbonatite of the east contact zone at Bayan Obo REE-Fe-Nb deposit, Inner Mongolia, China, is distinguished and analyzed for the first time. Compositionally, two subtypes of ancylite are identified as ancylite-(Ce) and calcioancylite-(Ce), with chemical formula of respectively: (Sr0.62,Ca0.18)0.8(Ce0.5,Nd0.36,La0.17,Pr0.02,Sm0.02)1.2(CO3)2(OH1.04,F0.14)1.2H2O (Ca0.73, Sr0.28,Ba0.17)1.2(Ce0.46,Nd0.15,La0.15,Pr0.04,Sm0.01)0.8(CO3)2(OH0.75,F0.06)0.8H2O which can be simplized as (Sr,Ca)2-x Cex (CO3) 2·(OH,F) 2-x·H2O.The carbonatite samples with ancylite are mainly composed of dolomite with calcite less than 10 persent. Ancylite disseminated within calcite or intergrowth of calcite and Ca rich dolomite, was not observed in main dolomite.