A new manganese oxide with formula Ba8Y2Mn5O21+δ (δ≤1) has been prepared by solid state reaction.The crystal structure and magnetic properties of the compound have been investigated by X-ray powder diffraction,selected area electron diffraction and magnetization measurements.The compound Ba8Y2Mn5O21+δ is well indexed by a rhombohedral lattice (Space group:R3m) with hexagonal lattice parameters o =5.7929(6) and c =28.586(4)(A).The atomic positions are determined by detailed structure analysis based on the structural characteristics of barium manganates and refined by Rietveld refinement.It indicates that the compound is a distorted perovskite with the Mn vacancies preferentially locating at the face-shared MnO6 octahedra (hexagonal perovskite unit) and Y at the apex-shared YO6 octahedra (distorted cubic perovskite unit).Thermal magnetization measurements reveal the presence of antiferromagnetic interaction between Mn ions in the compounds.However,the deviation from the Curie-Weiss plot at low temperature indicates the existence of localized magnetic coupling.