Granular activated carbon (GAC) was used as supporting medium for bacteria in anaerobic process. The effectiveness of using GAC was examined with laboratory-scale experiments. Synthetic wastewater containing phenol was converted to methane more successfully in fluidized bed using GAC. Fluctuation in influent concentration of phenol was intentionally made to examine the dynamic response of the process. Temporary increase of influent phenol from 400 mg/l to 1000 mg/l, 2000 mg/l, and even to 4000 mg/l for 4 days did not affect the effluent phenol as much as methane production because adsorption and desorption capacity by GAC functioned as stabilizer of the fluctuation. The adsorbed phenol was then gradually degraded into methane. Higher biomass concentration and lower adsorbed phenol were observed in the top part of the fluidized bed than in the bottom part.