Abstract: Using ⁵⁹Co as the simulated radioactive isotopes, the removal of Co²⁺ from the simulated radioactive wastewater with boric acid in nuclear power plant by in-situ synthesis of Mn₃O₄ was studied. The effects of reaction time, n(Mn²⁺): n(Co²⁺), air flow, reaction temperature and pH on the Co²⁺ concentration in the effluent were investigated. An orthogonal experiment table L₉(4³) was used to optimize the process parameters. When the wastewater contained 10 mg/L Co²⁺ and 1 000 mg/L boric acid (as B), the optimal process was as follows: the reaction time 105 min, n(Mn²⁺): n(Co²⁺)=25: 1, air flow 0. 7 L/min, reaction temperature 65 ℃ and pH 10. 5. Under the optimal condition, the effluent concentration of Co²⁺ was 5. 68 ng/L, the removal rate was more than 99. 99%. XRD analysis proved that the sediment was the mixture of Mn₃O₄ and CoMn₂O₄.