Microalgal biomass has a great potential in research for new alternative energy source. We can accelerate decomposition of microalgae cell wall with different pre-treatment procedures or with bioaugmentation. In our study we tested biogas production of microalgae mix, cultivated in anaerobic digestate. To accelerate hydrolysis and to approve efficiency of biogas production with microalgae as substrate, we started biological pre-treatment and bioaugmentation with cellulolytic thermophilic bacteria Clostridium thermocellum. We did two tests BMP in thermophillic anaerobic conditions (T=55 °C). At first experiment pre-treatment with C. thermocellum (10 %, v/v) lasted for 7 days and at second for 3 days. At both experiments the bioaugmentation was carried out once and three times with the same dose of C. thermocellum (10 %, v/v): on day 0, 3 and 6 of the bioaugmentation experiment. At first experiment we used 1,17 g TVS/g TVS microbial inoculum. Due to finding microbial biomass was owerloaded at first experiment, at second experiment we used 0,69 g TVS/g TVS microbial inoculum. In both experiments, biological pre-treatment of microalgae has no effect on methane production. At second test BMP biomethane potential of untreated microalgae was low (148,6 mL CH4/g TVS), while bioaugmentation with C. thermocellum increased methane production for 20 %. Furthermore, tree-times successive addition of bacteria increased methane production for 52 % compare to untreated microalgae. Regarding to results, bioaugmentation with C. thermocellum improved methane production from microalgae and represent efficient way to accelerate decomposition of microalgae cell wall.