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.