Co-production of syngas and hydrogen via chemical looping steam methane reforming (CL-SMR) using double perovskite-type oxide La1.6Sr0.4FeCoO6 as an oxygen carrier was studied. The reaction mechanisms, including the synergistic effects, the metal transitions, the oxygen diffusion and the migration of reaction boundary during the two-step reactions, were systematically investigated by the characterizations of the oxygen carriers at different reaction stages using XRD, XPS, H-2-TPR and TG technologies. Meanwhile, isothermal reactions were carried out in a fixed-bed reactor to analysis the reaction products. Three reaction stages including the total oxidation of methane with the active adsorbed oxygen, partial oxidation of methane with the lattice oxygen, and the methane decomposition were identified, using the surface of the oxygen carrier particles as reaction boundary. A large number of syngas was generated due to the concordant of methane dissociation with the lattice oxygen diffusion, and the resistant to coke formation was enhanced effectively. In the steam dissociation stage, the deep reduced metals (Fe2+ and Co-0) combining with the abundant oxygen vacancies provided enough active sites for the breakage of H-O bond of H2O. The oxygen vacancies were neutralized immediately by the O atom, and the two H atoms combined together to form amounts of H-2. These results suggested that, the positive roles displayed by the synergistic effects between multi-metals in double perovskite structure could effectively promote the partial oxidation of methane and steam splitting. It provides a potential way to develop more active oxygen carrier for CL-SMR to co-produce syngas and hydrogen by comprehensively considering the methane dissociation and the lattice oxygen diffusion. (C) 2017 Elsevier B.V. All rights reserved.