Denaturing gradient gel electrophoresis analysis of polymerase chain reaction-amplified genes coding for 16s rDNA was used to assess changes of biodiversity in a biotrickling filter used to treat waste air containing toluene. The results shown that along with gradually increase of removing capacity for toluene, microbial community structure in packing materials sampled from biotrickling filter also changed markedly over periods of experiment. Under selective pressure of toluene, the number of microbial species decreased but relative abundance of some predominant species increased，and microbial spatial location along the height of biotrickling filter tend to be identical.