The mechanisms of partial nitrification in sequencing batch reactor was studied under the condition of AOB oxygen affinity lower than NOB by calibrating the kinetic parameters of nitrification accurately. The measured DO half saturation constants for AOB and NOB were 0.46 and 0.14mg O₂/L respectively. Under the condition, the higher AOB specific growth rate than NOB was an important feature for achieving partial nitrification. The AOB and NOB specific growth rates were measured at 0.65 and 0.45 d^-1 respectively. The two-step nitrification model simulation indicated that the low DO and high SRT (sludge retention time) conditions were detrimental for achieving partial nitrification, which could be more readily achieved under the combined condition of relative high DO and low SRT. The experimental result verified the simulation results.