Abstract: Determining the most favourable recycle ratio is an important item in A²/O process operational optimization. In this work, feeding with sewage, the effect of recycle ratio on nitrogen removal when using different influent nitrogen loads was tested in an A²/O process, which available capacity was 52 L. Experimental results showed that increasing the internal recycle ratio from 0 to 6 produced a 14.0% increase in nitrogen removal. This increase was achieved by improving ρ(NO_x^--N) removal efficiency of 10.2 96 with an increase in ρ (NH₄⁺-N) removal efficiency of 3.8%. In order to stabilitate the effluent total nitrogen concentration and reduce operational cost, internal recycle ratio should be increasing with influent ρ(NH₄⁺-N) loading. The optimal internal recycle ratio is 2 in general.