Partial nitrification (PN) process is an essential pre-treatment to Anammox process for nitrogen removal from ammonium-rich wastewaters. High activity nitrifiers (HANs) are considered as the key to high-rate PN process. The HANs could be successfully enriched at free ammonia (FA) concentrations of 10-20 mg NH(3)-NL(-1), which differed significantly from those enriched at FA concentrations of 1-3 mg NH(3)-NL(-1). The maximum specific ammonium conversion rate, half saturation constant, half inhibition constant, cell yield and decay rate were 2.87 g N g(-1) VSS d(-1), 5.82 mg N L(-1), 8218.8 mg N L(-1), 0.15 g VSS g(-1) NH(4)(+)-N and 0.09 d(-1) for HANs at 30 degrees C, respectively, which endowed the PN process a volumetric conversion rate (VCR) as high as 2.22+/-0.33 kg N m(-3) d(-1). The cultivation mode of HANs was workable and the HANs were promising as inoculum for the PN process.