A numerical model of biofilm reactors was tested using Monod type biological kinetics in terms of the properties of feed solution and dispersion. The model is based on computer techniques and the numerical evaluation of the normalized biofilm mathematical model. The effect of inlet parameters including concentration, flow rate, mass transfer coefficient and biofilm thickness were investigated using the computer program developed. The numerical model was also applied to experimental data to demonstrate its validity. In thin biofilms kinetic coefficients and other parameters such as molecular diffusivity can be determined based upon experimental results. In thick biofilms, however, an additional kinetic parameter or any other quantity related to mass transfer is a prerequisite in the evaluation of experimental data.