The use of petroleum-derived products has given rise to environmental concerns regarding hydrocarbon pollution. Therefore, the development of innovative techologies for the clean-up of contaminated sites is a challenge. This thesis is an investigation of a three-phase fluidized bed biofilm reactor, as an effective technology for biological treatment of diesel-contaminated wastewater. The three-phase fluidized bed utilized in this research consists of support media (diameter of 600 um) with biofilm, and gas phase (air at 1.0 cm/s) in up flowing liquid (feedwater at 0.02 cm/s). The reactor influent is synthetic wastewater varying in COD concentrations in the range of 550-1300 mg/l and diesel concentrations between 70 and 200 mg/L. The results indicate that diesel fuel can be removed in the reactor with efficiencies up to 100% at a hydraulic detention time of 4 hours. Good quality effluent means a good reactor performance, where 55% of the diesel fuel was removed due to biological process.