The Mixed Inert Gas (MIG) produced by the novel Green OnBoard Inerting Gas Gen-eration System (GOBIGGS) mainly consists of carbon dioxide, nitrogen and oxygen. Because of the large solubility of carbon dioxide in jet fuel compared with nitrogen, the no gas release or equilib-rium model could not be employed any more. In this paper, first, a mathematical model of the ullage washing was set up to predict the variation of the oxygen concentration on ullage and in the fuel, and the gas evolution and dissolution rate were calculated by Fick's second law. Then, an experimental apparatus was constructed to verify the accuracy of the model. Finally, the numer-ical comparisons of ullage washing using Nitrogen Enriched Air (NEA) and MIG are presented under various flow rates and fuel loads, and the result reveals that the variation of the oxygen con-centration on ullage is nearly identical whatever the inert gas is NEA or MIG. However, the vari-ation of the oxygen concentration in the fuel is disparate, and the oxygen concentration decreases rapidly if the inert gas is MIG, especially when the fuel load is low or the flow rate of the inert gas is high. Besides, MIG could suppress the rising trend of the oxygen concentration on ullage when the aircraft ascends if the fuel tank is fully washed into an equilibrium state on ground.