Lack of systematic investigations on their dynamical and energetic stability preclude the full understanding of the structural determination and the corresponding mechanical properties for the semi-transition-metal borides ε2B (ε = Ti, V, Cr, Mn, Fe, Co, Ni, Nb, Mo, Ta, and W). The present energetic calculations show ν2B (ν = Ti, V, Cr, Nb, Mo, Ta and W) with the long-assumed I4/mcm symmetry are dynamically unstable, stemming from the large volume required to accommodate the boron atom at equilibrium. By freezing the unstable phonon mode, I4/m structure is predicted to be both energetically and dynamically stable. The structural stabilities for these compounds at finite temperature have been identified by the calculated Gibbs free energies and ab initio molecular dynamic simulation.