Aims: This study was carried out to investigate the effect of predators' growth on biological excess sludge reduction of adsorption/bio-oxidation process (A/B process) as a modification of activated sludge system. Materials and Methods: The real municipal wastewater after screening and gritting was pumped into A/B pilot plant which consists of two aeration and sedimentation tanks in series. The hydraulic residence time for A and B stages was set at 1 and 4 h, respectively, at an average flow rate of 32 L/h. During operation, the mixed liquor suspended solids (MLSS) in A and B stages was gradually increased. In this period, the operational parameters including dissolved oxygen, pH, volatile suspended solids (VSS), MLSS, alkalinity, biochemical oxygen demand (BOD5), chemical oxygen demand (COD), phosphorus, nitrogen, and sludge volume index were monitored. Results: The results showed that with increasing solid retention time (SRT) from 0.6 to 56.8 days in Stage A, the biomass yield (Y) decreased from 1.29 to 0.23 g VSS/g COD. Similar results were observed in Stage B and correspondence to 67% reduction of Y as SRT increased from 1.6 to 123.8 days. During the A/B operation, overall tBOD5and tCOD removal was 70% ± 20% and 57% ± 24%, respectively. Conclusion: Based on the results, A/B process operation with high SRT led to predator growth enhancement and lower biological excess sludge production.