The purpose of our current study was to clear the fate of organic carbon and nitrogen in seawater and sediment, especially the difference between organic carbon and nitrogen in dissolved and particulate forms and to determine carbon sequestration by seagrass ecosystem. We had carried out four types experiments: the diel variation for 24 hour, and seasonal changes of DOC (Dissolved Organic Carbon)/DON (Dissolved Organic Nitrogen) and POC (Particulate Organic Carbon)/PON (Particulate Organic Nitrogen) in outside and inside of seagrass communities, and the enclosed chamber experiment for evaluation of release of organic matters from seagrass exuadation and the sediment, decomposition experiments in laboratory and in situ. POM (Particulate Organic Matter) of seagrass leaf litter is resistance to the biological degradation under 25degreesC. (The rate constant of decomposition is 0.0083 d(-1), namely remineralization time is about 120 days.) While POM in water column shows POC is more rapidly consumed to PON, and POC/PON ratio decresed with time of decomposition. DOM in water column shows DON is more rapidly consumed to DOC and DOC/DON ratio increased with time of decomposition. This suggests that rate of solubilization of DON from PON may dominate a rate of remineralization of nutrients in seagrass communities. We developed a kinetic model of the seagrass decomposition process (SDP) changes in POC/PON ratio with time as well as phytoplankton decomposition model (Fujii et al., 2002). The SDP model was adapted to the results of three kinds of decomposition experiments where the initial C/N ratio and the volume ratio of POM and DOM differed by more than a factor of three. The apparent zeroth-order decrease in POC and the constant PON in the first labile decomposition process are well expressed by the SD model, and consequently, the calculated POC/PON ratio shows good agreement with the experimental result. A realistic model for CO2 sequestration was also developed using net organic production (net organic production=gross organic production-respiration), sedimentation inside seagrass bed and sedimentation at the open ocean (=export flux). We calculated net sequestration of carbon into deep water after 100 years under two different sinking velocities which are for 6 m.d(-1) and 24 m.d(-1). These results showed that 120 t-C for 6 m.d(-1) and 150 t-C for 24 m.d(-1) during 100 years were accumulated into deepwater, suggesting that 1.2 to 1.5 t-C-year(-1) for 31 t-C of gross production of seagrass km(-2) are accumulating into deep water. Seagrass ecosystem is very effective as one of the CO2 sequestration.