Abstract: Raw water samples were taken in bench scale, using AOB as target microorganism. The study investigated the oxidation efficiency of two oxidants to AOB by compared monochloramine (NH₂Cl), chlorine dioxide (ClO₂) with the combination. Berenbaum formula was introduced to judge the synergistic nature of the combined oxidants, and this article also studied the biological safety of filtration effluent oxidized by the combination of NH₂Cl and ClO₂. Results showed that there was certain efficiency when using NH₂Cl or ClO₂, respectively. When AOB was inactivated to a logarithmic level of 5.26, 1.5mg/L chlorine needed 30 minutes, while 0.225 mg/L chlorine dioxide only needed 15 minutes. It was demonstrated that to get the same inactivation rate of AOB, ClO₂ needed lower oxidation dosage and shorter contact time when compared to NH₂Cl. Compared with the individual oxidantion of NH₂Cl or ClO₂, NH₂Cl combined with ClO₂ had better oxidation efficiency to AOB. According to the Berenbaum formula, the combination of NH₂Cl and ClO₂ was synergistic to AOB. Oxidation experiment was also carried on simulative filtration effluent samples, results showed that the residual of ClO₂ was increased obviously resulted from the exit of NH₂Cl, which could prevent bacteria regrowth in distribution. Therefore, the combined oxidants of NH₂Cl and ClO₂ were better than NH₂Cl or ClO₂ since the oxidant dosage and byproducts were significantly decreased, followed by safer biclogical stability.