The thermodynamic stability, magnetism and half-metallicity of Heusler alloy Co2MnX(X = Si, Ge, Sn)(1 0 0) surface are comprehensively investigated from the first-principles calculations. The calculated phase diagram indicates that with increasing core electrons of X atoms in Co2MnX(1 0 0) the CoCo termination will be faded out of the thermodynamic equilibrium region gradually. Due to the difference of Co-X bonding the surface Co and Mn atoms prefer to move towards the slab and vacuum, respectively. By comparing with the bulk, the surface Co and Mn atomic magnetic moments (AMMs) are enhanced obviously because of the significant surface d-electronic localization. Further investigations of the partial density of states (PDOS) show that the half-metallicity observed in bulk has been destroyed by the surface states in deficient-Mn atomic terminated surface, only the terminations capped pure Mn atoms in Co2MnSi(1 0 0) and Co2MnGe(1 0 0) surfaces preserve spin-polarization of 100% instead of the Co2MnSn(1 0 0) surface, which is a possible explanation for low experimental tunnel magneto-resistance (TMR) value in Co2MnSn(1 0 0)-based magnetic tunnel junctions (TMJs). (C) 2012 Elsevier B.V. All rights reserved.