The impacts of Mg, MgO, CaO, SrO or BaO incorporation on La 2O3 MOS devices have been examined. Roll-off behavior in the flat-band voltage (Vfb) of the MOS capacitors on equivalent oxide thickness (EOT) has been observed. The roll-off characteristic has been suppressed with Mg incorporation. On the other hand, with MgO, CaO, SrO or BaO incorporation, the characteristic has been enhanced. Interface-state density (Dit) has slightly increased and leakage-current density (J g) has unchanged with the incorporation.