This paper evaluated the effect of sludge history on the efficiency of aerobic sludge stabilization. The fate of excess sludge in activated sludge systems is closely related to the nature of biomass sustained in the system. The composition of biomass greatly affects stabilization performance. In this study, the impact of sludge history was investigated for different sludge ages in the range of 1-35 days in a system treating typical domestic sewage. A biochemical model was adopted to define particulate COD components for selected sludge ages. For each selected sludge age, the progress of aerobic stabilization was evaluated in terms of the same parameters for a period of 30 days. Model simulation indicated that the active fraction of the biomass (X-H) varied between 0.13-0.40 depending on the sludge age of the systems at the beginning of the stabilization period. It also showed that the nature of the biomass corresponding to a selected sludge age was a key factor determining the stabilization efficiency.