The fuel acclimation process offers flexibility in microbial fuel cell (MFC) power generation behavior. Different concentrations (50-200mg/L) of phenol were used for adapting the activated sludge (AS), obtained from a local petroleum wastewater treatment plant and Ralstonia eutropha pure culture. Anodic biomass capable of oxidizing phenol substrate, using either AS inoculum microbial consortium or R. eutropha in the MFC system, has been a reflection of growth supportive functionality of phenol and 150mg/L as initial concentration was used in the experiments. For both types of inocula. The results of phenol and COD removals obtained for closed system configuration were compared with those under open circuit condition. The current production by AS and R. eutropha was improved through phenol acclimation process. The highest power density (PD) using either AS or R. eutropha was 11 and 5.8mW/m2, respectively. In terms of using olive oil mill wastewater as the anodic substrate, the behavior of phenol-acclimated R. eutropha was better than that of the synthetic type of wastewater, and the PD value was 7.8mW/m2.