A reactor model was developed for the wastewater denitrification in the fluidized bed biofilm reactors with sand and activated carbon particles as support media. The biofilm model proposed by Boaventura and Rodrigues was modified based on the reaction scheme of two consecutive zero-order reactions. The reactor model also includes mass transport of nitrate and nitrite through the liquid film around bioparticles. The effects of liquid-phase mixing in the reactors were also taken account for by assuming the plug flow and the perfect mixing. Thin biofilms formed on the sand support media were fully penetrated by both nitrate and nitrite. On the other hand, the partial penetrations of both species were predicted to occur in thick biofilms on the activated carbon support particles. The removal rates of nitrate and nitrite in the case of full penetration were determined by the biomass quantity of the biofilm, while the surface area of bioparticles determined those in the partial penetration. In the case of partial penetration, the effective diffusivity in the biofilm, the liquid-bioparticle mass transfer and the liquid-phase mixing in the reactor play an important role in determining the volumetric removal rate of nitrate and nitrite.