Transition metal borides (TMBs) have long been considered as potential superhard materials due to it have strong B-B covalent bond and TM-B bond. Many TMB_s which have been predicted as superhard material have layered structure with TM layers and B layers stacked alternately. Up to now, several TMB_s have been synthesized and declared as superhard materials. However, none of them demonstrated an asymptotic hardness larger than 40 GPa which is generally accepted as the threshold of superhard materials. However, there is a paucity of reports about why can not form superhard materials in TMB_s. In this paper,  rhenium diboride、tungsten diboride and molybdenum diboride are considered as the material for exploring the hardness mechanism. But the TMB_s, 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. We found that the chemical bonds between metal layer and boron layer play an important role in determining the hardness. In three materials, ionic character bonding is the major chemical bonding between TM atoms and B atoms which is thereason that this three materials can not be superhard material. Our results are helpful to understand the hardness mechanism and design superhard materials in TMBs.