Macroalgae have recently attracted attention as a possible feedstock for energy as well as for biogas. Macroalgae have higher productivity than terrestrial plants, and do not compete with crops for arable land but insight is still necessary on the biogas yield for an overall sustainable evaluation. The objective of this study was addressed to evaluate the effect mechanical pre-treatment (MW) on anaerobic digestion (AD) conversion of the brown algae Fucus vesiculosus washed ashore in the Gulf of Riga. The AD were carried out in batch tests at 37 °C, over an incubation time no longer than 21–25 days, in order to evaluate the biological methane potential (BMP). Two mechanical pre-treatment methods were tested including a biomass washing with TAP water and a further manual chopping up to a maximum size of 5 mm. Total solids (TS) and volatile solids (VS) tests were defined according to EPA standards [1]. The batch test-based BMP was further compared with the theoretical biological BMP from Buswell's formula. The results show the BMP is doubling up to 200 l CH4/kg TS if any pre-treatments method is applied. The thicker cell walls of the brown marine algae respect other macro algae types and as well respect fresh water macroalgae which inhibit the activity of fermentative bacteria during the hydrolysis process.