Impacts of dissolved oxygen (DO) and initial sludge concentrations on aerobic digestion for sewage sludge treatment were studied without adding alkalis. The MLVSS removal efficiencies were 50% and 47% in 20 days when DO concentrations were about 5.0 and 7.0 mg/L, respectively. Value of pH increased slightly in the first 6 days and then dropped constantly. The decrease of MLVSS appeared first quick back slow trend during the process and the OUR declination was similar. At the same DO level of 5.0 mg/L, the lower initial sludge concentration favored the efficiency of sludge stabilization. Both TN and TP of the supernatant were continually increasing and the ultimate concentrations were three to four times the initial concentrations. Results obtained from the present work could provide basic information for the aerobic sludge treatment process.