We have examined the stability of catalytic activity and the effect of supercritical water on the phase and structural properties of MnO2 through a catalytic supercritical water oxidation of phenol. From the experimental result, the conversion of phenol slightly declined during the first 6 h. It is suggested by X-ray diffraction analysis that the change of the activity of the MnO2 catalyst is caused by the transformation of the crystal structure to the Mn2O3 catalyst in supercritical water. In addition, it is found that the lattice oxygen in the catalyst could be involved in the oxidation and play a role as an oxidizer. The oxygen supplied to the oxidation system functions to compensate for the used lattice oxygen as well as the reactant as the adsorbed oxygen on the catalyst.