The　over-proliferation　ofnitrite-oxidizing　bacteria　(NOB)　is　one　of　the　main　causes　for　the　decrease　of　the　nitrogen　removal　efficiency　in　municipal　wastewater　partial　nitritation/anammox　(PN/A)　process.　Municipal　wastewater　PN/A　was　established　in　a　sequencing　batch　reactors　(SBR)　and　the　effect　of　periodic　nitrogen　loading　rate　(NLR)　fluctuation　on　nitrate　accumulation　in　the　PN/A　process　was　investigated.During　the　experimental　period,　influent　ammonium　concentration　varied　periodically　while　aeration　rate　and　duration　remained　constant,　inducing　a　variation　of　NLR　within　the　range　of　0.24~0.48kgN/(m3·d).　Effluent　nitrate　concentration　gradually　increased　under　the　variation　of　NLR,　resulting　in　the　decline　of　both　nitrogen　removal　rate　and　efficiency.　Followed　by　the　variation　of　NLR,　the　reactor　was　operated　at　a　stable　NLR,　which　was　failed　to　restore　the　nitrogen　removal　performance.　NOB　was　easier　to　compete　for　oxygen　at　low　bulk　ammonium　concentrations,　which　was　the　main　reason　for　the　unwanted　nitrate　accumulation.　Therefore,　a　stricterprocess　control　of　DO　andresidual　ammonium　concentration　was　essential　to　the　stable　operation　ofPN/A　systems.　©　2017,　Editorial　Board　of　China　Environmental　Science.　All　right　reserved.