Abstract: Optimization of temperature, pressure, retention time and oxidant dosage was studied in order to improve the efficiency of excess sludge treatment by supercritical water oxidation. Response surface methodology was applied with the removal ratio of chemical oxygen demand (COD_Cr) as the response value. It was shown that reaction temperature and reaction pressure were the key factors affecting the COD_Cr removal rate. Interaction was discovered among reaction pressure, temperature and retention time. The COD_Cr removal rate was mainly dependent on pressure when the temperature was less than 410 ℃ and the retention time was within 120 s. A maximum COD_Cr removal rate ranging from 97.72% to 98.32% was achieved under the optimized conditions, which were reaction temperature 434 ℃, pressure 29 MPa, retention time 278 s and oxidant dosage rate 216%.