The　success　of　combined　partial　nitritation　(PN)　and　anammox　process　treating　low-strength　domestic　waste-water　depends　on　achieving　a　stable　and　efficient　PN.　In　this　study,　desirable　PN　for　domestic　sewage　with　low　temperature　of　11.8-16.9　degrees　C　was　achieved　in　a　granular　sludge　reactor　operated　in　anaerobic/aerobic　(A/O)　mode.　Average　nitrite　accumulation　ratio　of　97.3%　was　obtained　with　an　effluent　nitrite/ammonium　ratio　of　1.2　for　influent　ammonium　of　39.3-78.7　mg.L-1.　Quantitative　microbial　analysis　and　activity　batch　test　showed　that　nitrite　oxidizing　bacteria　(NOB)　were　effectively　suppressed,　while　ammonium　oxidizing　bacteria　(AOB)　were　dominant.　For　the　efficient　suppression　of　NOB,　A/O　mode,　aerobic　phosphorus　uptake　and　granular　sludge　could　play　important　roles.　Furthermore,　high　AOB　activity　was　obtained　with　an　average　ammonium　oxidation　rate　of　11.6　mg　N.L-1.h(-1),　which　could　be　due　to　the　abundant　psychrotolerant　microorganisms,　increased　content　of　extracellular　polymeric　substances　and　relatively　high　dissolved　oxygen　condition　of　the　reactor.