Abstract: La₂O₃ catalysts with different grain sizes were prepared under hydrothermal condition. The structure activity relationship of La₂O₃ catalysts with different grain sizes were investigated by using in-situ XRD, Raman, FT-IR and H₂-TPR, O₂-TPD. The results showed that the La−O bond of the La₂O₃ catalyst showed a significant elongation with increasing temperature, which affected its adsorption and dynamic storage of O₂. When increasing the grain size up to 57.4 nm, the oxygen storage capacity of the La₂O₃ catalyst started to decrease, accompanied by the enrichment of surface oxygen species, especially superoxide species, on the catalyst surface, which led to the over-oxidation of CH₄ and products and reduced the C_{2 +} hydrocarbons selectivity. The L-La₂O₃ catalyst with a grain size of 52.3 nm had a suitable content of surface oxygen species and a high oxygen storage capacity at 750 ℃. It exhibited the best C_{2 +} hydrocarbon selectivity up to a CH₄/O₂ of 3 and a vacancy rate of 1.6 × 10⁵ mL/(g·h).