Molybdenum dinitride (MoN2) was recently synthesized at a moderate pressure of 3.5 GPa, and a layered MoS2-type structure has been proposed. However, our first-principles calculations of thermodynamic, mechanical and dynamical properties suggest that this layered R3mstructure is unstable. Therefore, stable structures of MoN2at pressures from atmospheric pressure to 100 GPa have been further examined by utilizing a widely adopted evolutionary methodology USPEX for crystal structure prediction. We find that the ground state of the MoN2system is a pernitride structure with space group P63/mmcwhich transforms to a P4/mbmphase above 82 GPa. Chemical bonding analysis shows that one could assign MoN2as Mo4+(N24–); i.e., Mo is formally a d2metal, in agreement with the experimental results of Wang et al. The presence of covalent N2dumbbells and strong bonding between Mo4+and N24–is the source of the superior mechanical properties of these predicted ultra-incompressible MoN2pernitrides