A synthetic wa stewater containing phenol a s sole substrate wa s treated in a 218 L upflow anaerobic sludge blanket (UASB) reactor at ambient temperature. The operation conditions and phenol removal efficiency were discus sed, microbial population in the UASB sludge wa s identified ba sed on DNA cloning, and pathway of anaerobic phenol degradation wa s propo sed. Phenol in wa stewater was degraded in an UASB reactor at loading rate up to 18g CODP (L•d), with a 1:1 recycle ratio, at 26±1℃, pH 710-715. An UASB reactor was able to remove 99% of phenol up to 1226 mgPL in wa stewater with 24h of hydraulic retention time (HRT). For HRT below 24h, phenol degradation efficiency decrea sed with HRT, from 9514% at 16h to 9318% at 12h. It further deteriorated to 8815% when HRT reached 8h. When the concentration of influent phenol of the reactor was 1260 mgPL (corre sponding COD 3000mgPL), with the HRT decrea sing (from 40h to 4h, corre sponding COD loading increa sing), the biomas syields tended to increa se from 01265 to 3108g P(L•d). While at 12h of HRT, the biomas syield was lower. When HRT was 12h, the methane yield was 01308 LP (gCOD removed), which was the highest. Throughout the study, phenol was the sole organic substrate. The effluent contained only re sidual phenol without any detectable intermediates, such as benzoate, 42hydrobenzoate or volatile fatty acids (VFAs). Ba sed on DNA cloning analysis, the sludge was compo sed of five groups of microorganisms. De sulfotomaculum and Clo stridium were likely responsible for the conversion of phenol to benzoate, which was further degraded by Syntrophus to acetate and H-PCO2. Methanogens la stly converted acetate and H2 PCO2 to methane. The role of epsilon-Proteobacteria was, however, unsure.