This study was to investigate effect of solid retention time (SRT) and initial ammonium concentration (NH₄⁺-N) on abundance of ammonia oxidizing bacteria (AOB) using activated sludge collected from aeration tank of a municipal wastewater treatment plant (WWTP). Real-time polymerase chain reaction was applied to monitor dynamics of AOB abundance. The resulted showed that SRT and NH₄⁺-N influenced on AOB abundance. It was found that biomass in the longer SRT and higher initial NH₄⁺-N concentration were higher than in shorter SRT and lower initial NH₄⁺-N. However, NH₄⁺-N oxidation rates in shorter SRT were higher than in longer SRT. After 168 days of operation, AOB remained abundance in all reactors but its abundance were lower than those in the starting sludge in low NH₄⁺-N reactors, whereas its abundance were higher than those in the starting sludge in high NH₄⁺-N reactors. The abundance of AOB were sensitive with NH₄⁺-N concentrations. The results of this study could be used as evidence for the influence of SRT and NH₄⁺-N on abundance of AOB and to some extent as reference for a design of nitrogen removal facility.