Molybdenum borides including α-MoB₂ and β-MoB₂ have been successfully synthesized from boron and molybdenum at high pressure and high temperature (HPHT). The crystalline structures are confirmed by Rietveld refinements in the hexagonal (P6/mmm) and rhombohedral (R-3m) crystal systems for α- and β-MoB₂, respectively. The values of Vickers hardness (HV) are 15.2 GPa for α-MoB₂, which is firstly obtained, and 22.0 GPa for β-MoB₂. The hardness results for α- and β-MoB₂ are in good agreement with theoretical values calculated by first-principle calculations. The difference in hardness between α- and β-MoB₂ is attributed to the puckered quasi-3D (three dimensional) boron layers in β-MoB₂ which is confirmed by the calculated results of the Electron Localization Function (ELF) and elastic constants. These results are helpful to understand the hardness mechanism and to design superhard transition-metal borides (TMBs).