The partial nitrification bioreactor (PNBR) was operated at a constant temperature and an influent synthetic wastewater pH of 35 degrees C and 8.5, respectively. The effects of dissolved oxygen concentrations (DO), hydraulic retention times (HRT), and the nitrogen loading rates (NLR) on the ammonium removal efficiencies and the NO2-N/NOx-N ratio were investigated. The activity of nitrite oxidizing bacteria was stimulated at high DO concentrations. The highest NO2-N/NOx-N ratio of 0.76 was obtained at the DO concentrations of 1.9 mg/L. The NLRs and HRTs significantly affect the NO2-N accumulation in the PNBR. Although the most of NH4-N (98%) was oxidized at the HRT of 10.3 h, the ratio of NO2-N/NOx-N [NO2-N/(NO2-N+NO3-N)] was too low (0.33). The removal efficiency of NH4-N descended to 92% while the ratio of NO2-N/NOx-N achieved the highest level of 0.74 at the HRT of 5.2 h. At the NLR of 830 g NH4-N/m(3). day, the ratio of NO2-N/NOx-N reached to the highest level of 0.76 under the operational conditions. However, the removal efficiency of NH4-N decreased from 94% to 92%. Further increased the NLR caused the deterioration of NH4-N oxidation and NO2-N/NOx-N ratio. The stable NO2-N production rate of about 810 g/m3. day (2.5 g/m(2). day) was obtained under the NLR and surface loading rate of 830 g NH4-N/m(3). day and 3.55 g/m(2). day, respectively.