Abstract: The microbial degradative characteristics of Di-n-octyl phthalate (DnOP) was investigated by strains Pseudomonas fluorescens FS1, Pseudomonas aeruginasa FS2 and Brevibacterium sp. FS3 that were isolated from the activated sludge at a petrochemical factory and a pyrochemical factory. The optimum pH of the 3 strains for degrading of DnOP is 6 5～8 0, 7 0～8 0 and 7 0～8 0; and the temperature is 20～35℃, 15～35℃ and 15～35℃, respectively. The degradation of DnOP by the 3 strains can be described by the first-order reaction model, with the primary biodegradation half lives of DnOP: Pseudomonas fluorescens FS1< Pseudomonas aeruginasa FS2 < Brevibacterium sp. FS3. The experimental results demonstrated that the strain Pseudomonas fluorescens FS1 has strong ability of degrading DnOP.