The olive oil industry in Morocco produces an important amount of olive mill wastewater (OMWs) which poses serious environmental problems, anaerobic digestion is the most interesting technique of olive mill wastewaters treatment. In this study anaerobic digestion of OMWs was performed in a semi continuous mesophilic anaerobic digester for 30 days, during the experiment, the volume of biogas produced was measured by the water displacement method. The pH was around neutrality throughout the experiment and the maximum of biogas yield coefficient was 2277 ml / g VS.