Effects of pressure on oxidative coupling of methane over MgO/BaCO3 catalyst were studied over the pressure range of 0.1 similar to 1.1 MPa at 1053 K. MgO/BaCO3 catalyst was deactivated at elevated pressures. Deactivated MgO/BaCO3 catalyst could not restore its activity in the experiment under atmospheric pressure. The results of XRD showed that a small amount of MgCO3 was formed in the deactivated MgO/BaCO3 catalyst. From the results of XPS analysis, we concluded that the BaCO3 migrated from the bulk to the surface of the catalyst occurred. SEM photos showed that the grain size of the deactivated catalyst increased. We suggested that these increased BaCO3 crystal grains, which had been inactive for oxidative coupling of methane, partially covered the active component of MgO in deactivated catalyst. This was also confirmed by the comparison of the surface areas of fresh and deactivated MgO/BaCO3 catalysts. The results of CO2-TPD indicated that the synergistic action of MgO and BaCO3, as well as the strongly basic sites of the MgO/BaCO3 catalyst were destroyed during the oxidative coupling of methane at elevated pressures. Furthermore, as compared with the fresh MgO/BaCO3 catalyst, O(2)(2-)concentration in deactivated MgO/BaCO3 catalyst decreased, and the oxygen diffusion rate also reduced. The latter conclusion was confirmed by CO pulse reaction.