Experiments on batch aerobic digestion in continuous aeration were carried out by using activated sludges acclimated to synthetic wastewater of glucose and peptone at various sludge ages. As sludge age decreased, VSS decreased quickly with the progress of digestion and the ratio of maximum VSS reduction to initial VSS increased, but VSS/SS was kept constant. The decomposition rate of sludge was proportional to the biodegradable VSS, and its proportional constant (rate constant of decomposition) increased as sludge age decreased. The oxygen uptake rate of sludge was related to the decomposition rate of sludge by using a reaction model where the composition of activated sludge cell was C5H7NO2. Moreover, the SVI of sludge increased with the progress of digestion, but the final SVI was within the range of good settleability.