Plant material decomposition experiment was carried out in room condition from 1998 to 1999 (lasted for 12 months) The plant material and top-layer sediment was collected from East Taihu Lake Jiangsu Province, China. Three ecotypes of aquatic plant were used:emerged plant (Zizania latifolia), floating-leafed plant (Nimphoides peltatum), and submerged plant (Vallisneria spiralis, Potamogeton maackianus) It resulted that, floatnig-leafed plant had the highest decomposition rate and annual reduction ratio. Submerged plant had high decomposition rate in the early stage and the lowest annual reduction ratio. The decomposition process of emerged plant was very lengthy. By the end of the experiment,more than 70% of the bio-mateiialwas reduced by decomposition for emerged and floating-leafed plan, but less then 50% for submerged plant. Phosphorus release was prior to TOC in the decomposition process. It means that most of the phosphorus in the plant material can be released to the water when aquatic plants die down in the winter Nitrogen was released from the plant material parallely to TOC reduction,and nitrogen content ratio in the plant material nearly kept constant in the decomposition process for emerged and floating-leafed plant.