Molten oxidation method is an environmentally friendly treatment technology for metallurgical slag, which has attracted an increasing amount of attention in recent years. The isothermal oxidation kinetics of iron-rich molten nickel slag in air atmosphere was studied. The actual melting temperature (liquidus temperature) of modified nickel slag was determined to be 1488 ℃ by high temperature confocal laser scanning microscope (HT-CLSM). The variation curve of Fe_²⁺ concentration with time during isothermal oxidation of molten nickel slag was obtained by isothermal oxidation experiment of tubular furnace. The oxidation reaction order of Fe_²⁺ in molten nickel slag is 1.45~1.26 and the apparent activation energy is 286.83 kJ/mol by differential method and Arrhenius equation. The oxidation kinetics of molten nickel slag is dominated by diffusion control. Based on the Whitman theory and Higbie theory, the diffusion coefficients of oxygen in the melt are estimated to be 2.02′10_^-9 m_²/s~4.42′1010_^-9 m_²/s and 0.75′10_^-9 m_²/s ~3.28′10_^-9 m_²/s, respectively.