Abstract: The anaerobic ammonia oxidation (Anammox) process, which involves the simultaneous conversion of ammonium and nitrite into nitrogen gas under an anaerobic condition, has recently been attracting attention. This is because it consumes less energy, produces less sludge, and requires fewer carbon sources for denitrification as compared to the conventional nitrification and denitrification process. However, the inhibition phenomenon was always found in an Anammox reactor when there are High concentration of NH4+-N and NO2--N feeding. A study was made of the effect of adding recirculation in an Anammox reactor to alleviate the high substrate concentration inhibition. The characteristics of nitrogen removal were investigated in two up-flow reactors by adding effluent recirculation and inoculating Anammox sludge under high substrate concentration. After 116 days running in the reactor 1 with effluent recirculation, the results showed that the effluent concentrations of NH4+-N and NO2--N only were 44 and 60 mg/L and nitrogen removal load was 7.87 kg N/(m3·d), when the influent concentrations of NH4+-N and NO2--N were up from 100 and 130 mg/L to 602 and 782 mg/L respectively. The transformation ratio of NO2--N removal，NO3--N production and NH4+-N removal were 1.303/0.24/1. On the other hand, after 67 days running in reactor 2 without effluent recirculation, the effluent concentrations of NH4+-N and NO2--N were 174 mg/L and 253 mg/L, and nitrogen removal load was kept at 4.31 kg N/(m3·d), when the influent concentrations of NH4+-N and NO2--N were up from 100 and 130 mg/L to 456 and 600 mg/L respectively. The transformation ratio of NO2--N removal，NO3--N production and NH4+-N removal were 1.303/0.21/1. It indicates that effluent recirculation had a high dilution function and was helpful in avoiding the inhibition effect of a high substrate concentration on the activity of Anammox. It had no effect on effluent recirculation on NH4+-N and NO2--N transformation in Anammox process. The granular size of sludge was mainly 1.25-2 mm in reactor 1, while in reactor 2, the granular size was 0.9-1.6 mm, showing that, effluent circulation increased the liquid up-flow velocity and improved the production of Anammox sludge. Therefore, effluent recirculation was beneficial to enhancing the nitrogen removal under high substrate concentration in a Anammox reactor.