Hydrothermal oxidation of high-level phenol containing wastewater was performed in a continuous reactor. Results show that COD removal rate increases with the increase of ratio of the amount of H2O2 supplied to the stoichiometric demand of H2O2, K; However when K is more than 1.5, the amount of H2O2 supplied does not affect COD removal rate evidently. Under sub-critical conditions, COD removal rate depends on temperature and phenol feeding concentration, and pressure does not have significant effect. Under supercritical conditions, COD removal rate depends not only on the above mentioned conditions, but on water density as well. The performance of different catalysts used in the process was compared. The experimental results show that Cu(NO3)(2) is the best one in the four catalysts.