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).