Effect of pentachlorophenol (PCP) on COD and ammonium removal rates was investigated, and also changes of bacterial community within aerobic granular sludge were monitored by terminal restriction fragment length polymorphism (T-RFLP) technique. Compared with the removal rate of COD, the rate of ammonium removal is more sensitive to PCP. As the ammonium and COD removal rates decreased consistently, the bacterial genera within the aerobic granular sludge also decrease with the increase of PCP concentration. According to the response of PCP, the bacteria could be categorized into four groups as follows: (1) The most sensitive organisms such as Microbacterium, Streptococcus and two new kinds of microorganisms; (2) The more sensitive microorganisms such as Corynebacterium, Nevskia. and another unreported bacterium; (3) The lower sensitive organisms encompassing Mycoplasma, Exiguobacterium, and another microorganism which has not been reported yet; (4) The PCP-tolerant organisms, including microorganisms indicated by 13 TRFs are represented as 69, 71, 82, 175, 198, 241, 229, 232, 233, 240, 245, 269 and 449bp. To most noticeable, TRF relative areas of the microorganisms indicated by TRFs 82 bp and 175 bp achieve 22.7% and 13%, respectively, and become predominant within the aerobic granular sludge with the PCP concentration at 30 mg/L.