A fluidized bed bioreactor has been used to carry out biological denitrification in the present work as it offers many advantages over other types of reactors. Plastic beads of low density (1055 kg/m3) were used as fluidizing media with Pseudomonas stuzeri, a denitrifying microorganism immobilized on them. The denitrification rate under different operating conditions was studied. The results of experimental results were used to simulate a biofilm model. The effect of initial concentration of nitrate, liquid flow rate and biofilm thickness on biomass concentration of reactor was studied. The concentration of nitrate at biofilm surface was found by iteration. Knowing the concentration at biofilm surface, the external mass transfer coefficient for the process was calculated for each run. The effect of parameters like liquid flow rate and biofilm thickness on external mass transfer coefficient was studied. A functional relationship between Sherwood, Reynolds and Schmidt numbers was established.