Although HfB₁₂ is a promising surperhard material because of the boron cuboctahedron cage, the Vickers hardness of HfB₁₂ remains controversial. We apply first-principles calculations to investigate the influence of a transition metal on the structural stability, Vickers hardness and thermodynamic properties of HfB₁₂. The Vickers hardness of HfB₁₂ is 39.3 GPa. In particular, the Vickers hardness of TM-doped HfB₁₂, which are novel superhard materials, is larger than 40 GPa. The Vickers hardness of Re-doped HfB₁₂ is up to 47.6 GPa. The improvement of Vickers hardness is that the introduction of an alloying element improves the localized hybridization between B and Hf, and then enhances the bond strength of the B–B covalent bond and the Hf–B bond. In addition, these alloying elements enhance the melting-point and Debye temperature of the HfB₁₂. Therefore, we believe that alloying is an effective method to improve the Vickers hardness and thermodynamic properties of HfB₁₂ superhard material.