Rice straw was converted into biogas using a high-rate anaerobic digestion system, anaerobic-phased solids digester system (APS-digester system). The system was stable and, with proper design, could become a space-efficient, high-rate solids digestion system. Ammonia is used as a supplemental nitrogen source for rice straw digestion. The effects of different pretreatment methods, physical (mechanical), thermal and chemical (ammonia) treatment, on the digestion of rice straw were investigated at the mesophilic temperature of 35 degrees C. A combination of grinding (10-mm length), heating (110 degrees C), and ammonia treatment (2%) resulted in the highest biogas yield, 0.471 g(-1) VS-1 fed, which is 17.5% higher than the biogas yield of untreated whole straw. Pretreatment temperature has a significant effect on the digestibility of straw.