Spherical NbB_2−x particles were prepared by increasing the intrinsic B/Nb stoichiometry (x) during self-propagating high-temperature synthesis. Because of the strong anisotropy of the chemical bonding, the shapes of the hexagonal-structured transition metal diborides are generally reported as a hexagonal plate and prism, and this is the first time for the hexagonal-structured transition metal diborides to display the spherical morphology. Moreover, this paper suggested a technical and theoretical approach to synthesize the spherical particles for the compounds with wide stoichiometric ranges.