The density functional theory has been employed to investigate the geometrical structures, relative stabilities, electronic and magnetic properties of Nb n O (n=3–10) clusters. The ground state structure are obtained by adding one O atom to the Nb n clusters or by replacing one Nb atom by O atom in Nb n +1 clusters. The results of binding energy and the second difference in energy show Nb 7 O and Nb 9 O have strong stability. According to the natural population analysis, O atom attracts about 0.8 e charges from its neighboring Nb atoms. The magnetism of the Nb n O clusters mostly stems from the contribution of 4d orbital of Nb atoms.