The effects of shock loading on the performance of a nitrifying biofilm reactor were investigated over a wide range of conditions by varying the feed concentration of ammonium (S-0) and dilution rate (D) respectively. It was found that variation in the effluent ammonium concentration as S-0 or D changes was-subject to a semi-U shaped curve. The response patterns of the nitrifying biofilm reactor to one-step change in S-0 at a constant dilution rate, and to one-step change in D with keeping constant S-0 have no significant difference.