In this study, enhanced removal of nitrogen from real domestic wastewater with low organic carbon to nitrogen ratio of 2.1-3.6 was achieved in a sequencing batch biofilm reactor (SBBR) with enriching anammox bacteria in situ. The SBBR was inoculated with ordinary nitrification sludge and operated under alternative anaerobic, aerobic and anoxic patterns. During the start-up period, influent nitrogen and carbon loading rate gradually increased. According to the quantitative microbial analysis, the growth of ammonium oxidizing bacteria and anammox bacteria were promoted, resulting in an increase from 6.56 x 10(4) to 1.51 x 10(7) and from 1.11 x 10(4) to 1.69 x 10(6) gene copies.mL(-1), respectively. During the steady period (Day 102-205), nitrogen removal efficiency of 88.2% was achieved with an effluent total nitrogen of 6.3 mg NI-1 for average influent ammonium of 53.9 +/- 4.0 mg.L-1. Nitrogen removal was steady when temperature decreased from 27.0 to 12.8 degrees C. Variations in nitrogen compounds suggested that anammox reaction could be the main pathway for nitrogen removal, according to the results of activity batch and continuous tests. For the high nitrogen removal performance, biofilm and post-anoxic unit could play important roles. Overall, this study provided an alternative method to improve the removal of nitrogen from sewage by directly enriching anammox bacteria.