Weight loss patterns in the decomposing leaves and sheath of seagrass, Thalassia hemprichii - were studied in relation to their organic carbon and nitrogen dynamics in a seagrass bed of Bise, Okinawa using litter bags - Comparisons were made between seagrass leaves and sheaths at - different seasons; Spring, Summer and Autumn 2007. The weight of sheaths, with low initial N concentration, decreased during the first 2 weeks of the experiment, and then remained relatively constant. On the other hand, the weight of leaves, with a high initial N concentration, decreased gradually throughout the experiment. In addition, the total nitrogen content in the decomposing leaves and sheaths varied seasonally. The weight loss patterns of leaves and sheaths were low in autumn, and high in summer after 1st week of the experiment. The total nitrogen content of leaves was higher in summer than in other seasons. During the experiment, the TN concentration increased and the TOC concentration decreased with time. This caused the C/N ratio to decline also with time. T. hemprichii biomass showed seasonal and spatial variation with regards to density and shoot height. The net losses of both carbon and nitrogen were 12.831 g m^-2 and 0.634 g m^-2 respectively at St. 3 which had the highest T. hemprichii biomass, and 1.228 g m^-2 and 0.057 g m^-2 respectively at St. 1 which had the lowest biomass. The estimated amount of TOC and TN from litter differed between plants parts. Therefore, it is necessary to consider not only leaf but also sheath as a nutrient source due to their differing nutrient values and decomposition processes.