Pre-treatment of raw materials is a necessary procedure to improve biogas production in anaerobic process. The aim of this study was to determine the effect of mechanical pre-treatment on the quantity and quality of biogas of methane fermentation process. The experimental material consisted of energy grasses: Miscanthus giganteus and Phalaris arundinacea L., of two different ranges of chaff length: 0.5-3 mm and 4-8 mm. The success of the experiment indicates that mechanical fragmentation has a significant positive effect on the anaerobic digestion process.