A study was done on the rate of oxygen consumption by a mixed culture microbial film in an aerobic fluidized bed biofilm reactor. For the description of oxygen consumption rate in the reactor where oxygen was a flux limiting component, a mathematical model was obtained and tested by an experiment performed with mixed culture microbes and a synthetic waste water containing glucose as the only carbon source. The experimental result showed that oxygen remains as a critically penetrating component as far as the inlet glucose concentration is above 5.4 times that of oxygen.