Autotrophic nitrogen removal is an innovative and economical nitrogen removal technology, in which partial nitrification (PN) is the key component. It is necessary to clear the impact of CuO nanoparticles (CuO NPs) on PN process since the wide application increased their opportunity for entering into wastewater. In this study, the short-term and long-term effects of CuO NPs on ammonia removal, extracellular polymeric substance (EPS) production and microbial community were investigated. Results suggested that CuO NPs in 1 mg L-1 had slight effect on PN, while that in 3-10 mg L-1 enhanced ammonia oxidation in the short-term exposure. In the long-term exposure experiment, CuO NPs with low concentration (<= 5mg L-1) improved both the bioactivity and relative abundance of ammonia-oxidizing bacteria (AOB). The suppression threshold on PN process of CuO NPs in short-term exposure was 30 mg L (1), while that in long-term exposure decreased to 10 mg L-1 due to the adsorption and accumulation of NPs. Nitrosomonas was the predominant AOB in PN reactors, it showed better tolerance to CuO NPs and performed the best performance when CuO NPs was 5 mg L-1. Long-term exposure within CuO NPs led to organisms death and resulted in the decrease of EPS. (c) 2017 Elsevier B.V. All rights reserved.