We present first-principles based calculations of electronic structures and tunneling conductance of Co 2 YZ / MgO͑ϳ2 nm͒ / Co 2 YZ͑001͒ ͑YZ = MnSi, CrAl͒ magnetic tunnel junctions ͑MTJs͒. It is found that YZ͑MnSi and CrAl͒-terminated interfaces are thermodynamically stable as compared with Co-terminated interfaces in Co 2 YZ / MgO͑001͒ junctions. In the CrAl-termination, no interface states appear in both sides of the junctions, preserving the half metallicity of Co 2 CrAl, while reduction of the spin polarization is significant in the MnSi termination. Co 2 CrAl, however, has no ⌬ 1 band at the Fermi level, thus the majority-spin conductance of Co 2 CrAl/ MgO/ Co 2 CrAl͑001͒ MTJs in the parallel magnetization is much smaller than that of Co 2 MnSi/ MgO/ Co 2 MnSi͑001͒ MTJs. We propose that MTJs having an ultrathin Co 2 CrAl layer between Co 2 MnSi electrode and MgO barrier, i.e., Co 2 MnSi/ Co 2 CrAl/ MgO͑001͒ junctions, can derive both the halfmetallic character at the interface and the large tunneling conductance through the ⌬ 1 channel in the parallel magnetization.