Abstract: The effect of different organic load rates (OLR) on ammonia-oxidation rate (AOR) of simulated tannery wastewater was investigated in the aeration device, using the activated sludge from a tannery wastewater treatment plant as the seed sludge. The results showed that the removal of ammonium (NH₃-N) was 99.6% after 10 days under zero OLR. Meanwhile, AOR gradually increased to 5.6 mg/(gh), which was 10 times greater than that at OLR of 6.0 g/(Ld). AOR remained at over 5.5 mg/(gh), and NH₃-N and COD were removed simultaneously when OLR was no more than 2.4 g/(Ld). However, AOR was declined remarkably under OLR of 3.0 g/(Ld), which made a negative impact on the oxidation removal of NH₃-N.