Abstract: In order to understand the influence of dissolved oxygen on sludge stabilization in auto-heated thermophilic aerobic digestion systems, the oxidation reduction potential (E_h), efficiency of cell lysis and sludge stabilization were studied at different digestion temperatures (45,5 and 65 ℃) with different sludge types (from a primary sedimentation tank, a secondary sedimentation tank and mixed sludge). The results showed that E_h remained below 0 mV at the initial stage of digestion, despite sufficient aeration supply. With digestion time prolonged, the ATAD system was not always running in the aerobic regime during the entire digestion process. Instead, it was operating in either anoxic or anaerobic condition at the initial stage, followed by aerobic digestion at the later stage. Compared with the other two digestion temperatures (45 and 55 ℃), cells in sludge digested at 65 ℃ were easily broken and lysed, which resulted in high concentrations of soluble COD (SCOD) and phosphorus in the sludge supernatant. The concentrations of SCOD at 65 ℃ were still high-up to 13,8 with TP of 126.4 and orthophosphate of 106 mg/L at the end of digestion, which showed that cell lysis can take place easily at the thermophilic temperature and oxygen-deprived condition. Although E_h values were generally low at the initial digestion stage, the VSS removal rates still remained greater than 38% in the first 15 days of digestion, which meets the Standard of Class A in Item 503 enacted by US EPA. This suggests that the stabilization efficiency for sludge treated by the ATAD process was not affected during the initial stage of anoxic and anaerobic digestion. Therefore, the aeration level can actually be reduced to a proper range to promote engineering applications of the ATAD process.