The natural abundance of stable isotopes is a powerful tool for evaluating biological reactions and process conditions. However, there are few stable isotope studies on the wastewater treatment process. This study carried out the first investigation on variations in natural abundance of carbon and nitrogen stable isotope ratios (δ13C and δ15N) of activated sludge in four types of advanced treatment process (extended aeration activated sludge (EAAS), aerobic-anoxic-aerobic (A2O), recycled nitrification-denitrification (RND), and modified Bardenpho (MB)) at a municipal wastewater treatment plant. The δ13C and δ15N values of influent suspended solids settled in the primary sedimentation tank (i.e., primary sludge) ranged from -25.4‰ to -24.6‰ and 0.5‰-2.9‰, respectively, during monitoring periods. The δ13C values of the activated sludge were -24.6‰ to -23.6‰ (EAAS), -25.4‰ to -24.3‰ (A2O), -25.7‰ to -24.9‰ (RND), and -25.7‰ to -24.3‰ (MB). The δ13C values of the activated sludge were similar to those of influent suspended solids. However, the δ13C values of activated sludge in EAAS was significantly higher than in A2O, RND, and MB. Meanwhile, the δ15N values of activated sludge were obviously higher than influent suspended solids; 5.8‰-7.5‰ (EAAS), 6.6‰-8.1‰ (A2O), 5.5‰-7.5‰ (RND), and 5.3‰-7.6‰ (MB). Changes in δ13C and δ15N values of the activated sludge within the treatment system were also found. These findings indicate that changes in δ13C and δ15N values of the activated sludge rely on important function for biological wastewater treatment such as nitrification, denitrification, and methane oxidation through wastewater treatment over time.