Sludge and organic solid waste are significant pollutant sources that cause much concern in big cities such as Hanoi and Ho Chi Minh city. In this study, thermophilic anaerobic digestion at 55°C was choosen for cotreatment of the above two types of wastes. The experiment was carried out in a 1000 litre volume reactor fed with a mixture of septic sludge and organic waste at 1: 1 ratio (vol/vol). The obtained results showed that the produced biogas contained high ratio of CH4 (above 70 percent) as soon as pH in the reactor was stabilized at 7 -7.2 and microbial seeding previously adapted to the fennentation conditions was added. After 50 days of operation, 80.7 percent of COD from the waste was removed. Density of methanogens in the reactor increased over the time, and the methanogenic community changed from that dominated by hydrogenotrophic species (such as Methanomicrobium) at early stages of the treatment to that dominated by acetoclastic species (such as Methanothrix) at later stages. The ratio of methane in biogas at later stages of the digestion reached 60-70 percent. Output from the present study would serve as the scientific arguments for control of anaerobic digestion process, as well as for potential application of thennophilic anaerobic digestion in the treatment of sludge and organic solid waste, aming at energy production and environmental protection in the near furture.