The anaerobic ammonium oxidation (Anammox) was successfully started up in an upflow biofilm (UBF) reactor and operated for 435 days at 35 degrees Celsius. The process development could be divided into four phases, i.e. endogenous denitrification without Anammox reaction (P1), functional biomass enrichment under relatively low nitrogen loading rate (NLR) (P2), specific Anammox activity (SAA) enhancement with high NLR (P3) and the final stable stage (P4). The maximal NLR was as high as 34.5 kg N/m(3)d. Nitrogen mass balance showed that 88.84% of input nitrogen converted to dinitrogen gas, 1.74% was used for cellular synthesis and the rest was converted to nitrate. The final produced biomass was fast-growing with SAA of 1.8 g N/gVSS d and doubling time of 4.3-7.4 d. Granulation and biofilm formation contributed to the biomass enrichment, while appropriate recirculation as well as relatively high temperature (35 degrees Celsius) helped to promote SAA. Effective retention time and promoted sludge activity were considered as the key factors for bacterial growth and efficient Anammox process.