The modern iron and steel industry produces large emissions of CO₂ annually. Electrolytic reduction of molten slag containing iron oxide at high temperature using an inert oxygen evolving anode is an alternative process to reduce or eliminate the formation of CO₂. In order to establish reasonable process parameters of electrolytic method for steel containing Ni, it is necessary to master the electrochemical behavior of Ni²⁺ in molten slag. However, investigations on the electrochemical behavior of Ni²⁺ in molten slag at higher temperatures were very limited, which can probably be attributed to the experimental difficulties associated with the operation of high-temperature electrochemical cells. An electrolytic cell with a controlled oxygen flow and Pt, O₂(air)|ZrO₂ used as reference electrode was constructed integrally through a one-end-closed magnesia partially stabilized ZrO₂ solid electrolyte tube. Electrochemical behavior of Ni²⁺ on Ir electrode was investigated in SiO₂-CaO-MgO-Al₂O₃ molten slag at 1673 K by means of electrochemical techniques such as cyclic voltammetry (CV), square wave voltammetry (SWV), chronopotentiometry (CP) and potentiostatic electrolysis. The results show that both diffusion in the molten slag and electromigration in the ZrO₂ solid electrolyte for the O^2- are not rate-determining steps of electrochemical reduction reaction process of electroactive ions. It is feasible to study electrochemical behavior of Ni²⁺ in the molten slag with the aid of the electrolytic cell with a controlled oxygen flow under the present experimental conditions. The reduction of Ni²⁺ on the Ir electrode in the molten slag is found to be a reversible reaction with a single step, and the rate of the process is diffusion controlled. Two diffusion coefficients of Ni²⁺ in the molten slag containing 3%NiO derived respectively from CV and CP are (3.50±0.18)×10^-6 and (2.80±0.22)×10^-6 cm²/s, which are consistent with records in the relevant literatures.