We investigated the degradation of three fractions of matured rice plant (stem, leaf and root) in paddy soil. To determine the loss of dry weights of three fractions due to degradation, 0.5g of each fraction was sealed in a nylon bag (mesh size 25µm) and buried in a glass bottle containing the mixture of 400 g soil and 130 ml water. The loss of dry weight of stem and leaf was 60% and 50%, respectively, after incubation at 30°C for 50 days. Root was the most refractory against degradation, showing only 25% loss of dry weight. For the determination of dehydrogenase activity and the production of CO2 and CH4 accompanying with the degradation of polysaccharides in the plant, each fraction of 1.5g was sealed in a glass bottle with the soil-water mixture prepared as same as above. The bottles were incubated at 30°C for 30 days without shaking. The highest microbial activity was observed in the bottle containing the stem, in which the microbial population reached the stationary state at 11th day during the period of incubation. In the bottle containing the leaf, the stationary state of the population was realized at 25th day during incubation. In both bottles, CH4 formation reached the maximum at 11th day, then decreased. In the bottle containing the root, though microbial activity kept slowly increasing throughout the period of incubation, CH4 formation from root was not observed. The production of CO2 was observed prior to CH4 formation in all bottles. Investigation is currently underway on the fraction-specific changes in the composition of organic matter due to degradation of rice plant. This report includes a part of the results from research conducted under with Aomori Prefectural Government, Japan.