A reactor model was developed for the simultaneous nitrification and organic oxidation of wastewater in airlift reactors with bioparticles. The diffusion and reaction in the biofilm of bioparticles, liquid-solid mass transfer, and gas-liquid oxy-gen transfer were taken into account together with the consideration of the nitrification and organic oxidation by the biomass suspended in the liquid phase of the reactor. Double Monod-type kinetics was employed for representing the reactions of the nitrification and the organic oxidation. Using the numerical simulation, it was discussed how the nitrification and the organic oxi-dation of the wastewater were influenced in the airlift reactor by effective diffusivity in the biofilm, bioparticle media size, air velocity and reactor operation pressure, flow rate and organic concentration of the feed, and maximum organic oxidation rate in the biofilm. The oxidation rate of ammonia and nitrite increased at the given conditions of the air velocity, the media amount and the biofilm thickness, as decreasing the media size. It was demonstrated in the simultaneous nitrification and organic oxidation of wastewater that the nitrite oxidation was first affected by the reactor liquid-phase concentration of dissolved oxygen and the organic oxidation was influenced by the biomass suspended in the liquid phase of reactor. As increasing the organic concentra-tion of the feed, the organic oxidation rate increased but the oxidation rates of ammonia and nitrite decreased.