A novel moving anaerobic biofilm reactor was used to treat whey wastewater. In this process, biofilm was grown on a plastic biofilm media module, which was vertically moved up and down in the bulk fluid. The objectives of the study were to investigate the mixing and performance characteristics of the new process in treating whey wastewater. The mixing efficiency was indicated by a dispersion number, D-L/uL. D-L/uL was up to 1.34, showing that the anaerobic reactor can be taken as a completely mixed reactor. At mesophilic conditions (35 L 2 C), the admissible volumetric COD loading rate up to 11.6 kg COD m(-3) day(-1) was achieved with the COD removal efficiency of 89% and the hydraulic retention time (HRT) of 1 day. When the HRT was 0.6 days, the volumetric COD loading rate was 15.2 kg COD m-3 day(-1), but COD removal efficiency decreased to 81 %. The percentage of methane (CH4) in the biogas was 63% on average and the yield of methane was 333.4 L CH4 kg(-1) COD removal at ambient conditions.