A conventional laboratory scale annular reactor was employed to investigate the dynamics of nitrifying biofilm growth. A dense and thin nitrifying biofilm was developed in this study. The results showed that the active growth of the nitrifying biofilm can be characterized best by the increase of the specific substrate removal rate until a maximum value, at which the maximum active film thickness and active biomass were attained. It was found that non active biomass accumulation directly resulted in the reduction in the specific ammonium nitrogen removal rate, however the ammonium nitrogen surface removal rate was not affected by the additional biomass accumulation. As a result, the credibility of the classic method using the substrate surface removal rate to estimate the maximum active film thickness was doubted. It was expected that thinner nitrifying biofilm ranging from 20 to 30µm has a higher specific nitrification rate to be 9.0mgNH4+-N/mgCOD-biomass/day.