Transition metal borides (TMBs) have long been considered as potential superhard materials. But the TMBs, which has been synthesized, do not exhibit superhard properties. In order to explain the reason why the synthesized TMBs do not exhibit superhard properties and to explore the hardness mechanism of TMBs, In this paper, molybdenum diboride is considered as the material for exploring the hardness mechanism. Through comparing the theoretical calculation and the existing data, we found that the chemical bonds formed between Mo and B atoms possess ionic character in MoB₂ which leads to it can only be a hard material. Our results are helpful to understand the hardness mechanism and design superhard materials in TMBs.