Abstract: In this study, laboratory-scale experiments were conducted by using Sequencing Batch Reactor (SBR) activated sludge process to treat wastewater stream from a pharmaceutical factory. Biological nitrogen removal can be achieved via partial nitrification and denitrification and its efficiency was above 99% at 23℃±1℃. The experimental results indicated that the nitrite oxidizers were more sensitive than ammonia oxidizers to the free ammonia in wastewater. The ammonia was oxidized at pH 7 and 7.5 respectively, which suggested a critical free ammonia concentration of 0.2mg•L^-1 at which the nitrite oxidizers were inhibited.