A mathematical model of the simultaneous removal of organic substances and nitrogen compounds in a biofilm reactor was developed, taking account of the relationships between oxidation, nitrification, denitrification, and mass transport. The model consists of a set of simultaneous mass balance equations for organic substances, ammonium nitrogen, oxidized nitrogen, oxygen, and alkalinity within the biofilm, and another set of transport equations for these substances within diffusion layer of the biofilm. Synthetic dual and triple Monod-type kinetics describing, respectively, oxidation, and nitrification and denitrification were incorporated into the mass balance equations. The model was evaluated based on experimental data and the computer simulations. This model successfully explained the mechanism of simultaneous removal and the effects of bulk concentrations of organic substances, DO, ammonium nitrogen, and alkalinity on simultaneous removal in a submerged biofilm reactor.