We study thermodynamic stability, mechanical properties, and microscopic mechanisms of transition metal monoboride TMB (take TiB, VB and CrB in 3d series; ZrB, NbB and MoB in 4d series; HfB, TaB and WB in 5d series as examples) by first-principles calculations based on density functional theory and plane pseudopotential wave method. We found thermodynamic stability and hardness anomalies of transition metal monoborides. In particular, as valence electron concentration is 8 e·(f.u.)^-1, thermodynamic stability is the most stable and hardness is the highest. To reveal its mechanism, we calculated electronic structure of TMB. As valence electron concentration of TMB is at 8 e·(f.u.)^-1, covalent bonding of pd blocked effectively dislocation slipping between metal bilayers, prevented shear deformation, and resulted in high hardness. These discoveries may help new superhard material designs.