Chemical looping combustion (CLC) is attracting attention as a promising technology for carbon dioxide separation. Oxygen carrier materials are the most important factor in determining the efficiency of CLC. In this paper, we present MgMnO3-delta with a defect cubic spinel structure as an oxygen carrier material with high efficiency, low cost, and stable properties. MgMnO3-delta has a non-stoichiometric oxygen content of MgMnO2.68. The reduced composition of MgMnO3-delta in H-2, CO, or CH4 is a complete solid solution of (Mg, Mn) O. The redox reaction from MgMnO3-delta to (Mg, Mn) O is reversible and stable. Interestingly, MgMnO3-delta also shows a chemical looping oxygen uncoupling (CLOU) phenomenon and is reduced to MgMn2O4 and MgO in N-2 at 900 degrees C. The oxygen transfer capacities of MgMnO3-delta under 15% H-2/N-2, 15% CO/CO2, 15% CH4/N-2, and 15% CH4/CO2 are 8.34, 8.56, 8.42, and 8.74 wt%, respectively, at 900 degrees C. The oxygen transfer capacity was well maintained during the 5th redox cycle. No phase or structure changes or agglomeration occurred as the redox cycle number increased. Moreover, MgMnO3-delta has a high tolerance to carbon deposition when used in CO or CH4 fuels.