Abstract: ⁵⁸Co and ⁶⁰Co are the primary radionuclides and contributors to collective dose of PWRs, which may exist in ionic, particle and colloidal state in the primary loop. The pore diameter of the most advanced filter used in chemical and volume control system (CVCS) in domestic PWRs is 0.1 μm, the removal efficiency of Co colloids with the diameter of less than 0.1 μm is very low. Activated corrosion products such as Co colloids may deposit on the surface of the primary loop and auxillary systems during shutdown, contributing to collective dose. The chemical conditions of primary loop during the oxidation operation process greatly affect the removal efficiency of radionuclides such as Co colloids. To achieve the best dissolution and removal effect of Co colloids, the dissolution behavior of Co colloids need to be studied. In this paper, simulated Co colloids were first synthesized in the laboratory. The average particle size of simulated Co colloids was characterized by TEM to be 40-50 nm. The composition of simulated Co colloids was characterized by TEM, SEM and EDS as CoO and Co. Then the effects of temperature (60-80 ℃), Li concentration (pH, 0.1-3 mg/kg), H₂O₂ concentration (0.20 mg/kg) and reaction time (0-6 h) on the dissolution behavior of Co colloids were investigated on simulated chemical condition of the oxidation operation process. The results show that the dissolution of Co colloids is quite fast, reaching reaction equilibrium in 0.5-1 h. The dissolution behavior of Co colloids is affected by temperature, Li concentration and H₂O₂ concentration. The dissolution of Co colloids is promoted by addition of H₂O₂ due to the weak acidity and strong oxidizing power of H₂O₂. However, too much H₂O₂ is unfavourable for the dissolution of Co colloids, which may be due to the possible formation of passive film on Co colloids. Low Li concentration or low pH is favourable for the dissolution of Co colloids. The oxidizing power of H₂O₂ is stronger at higher temperature promoting the dissolution of Co colloids. Yet high temperature is not favorable for the dissolution of Co colloids, as the dissolution of Co colloids is an exothermal process. Therefore a moderate temperature is the best choice for the dissolution of Co colloids. To summarize, the best chemical condition for the dissolution and removal of Co colloids during the oxidation operation process is at 70 ℃ with as low Li concentration as possible and about 10 mg/kg H₂O₂. The results in this paper provide important references for the improvement of the oxidation operation process of PWRs.