In the grassland ecosystem dominated by Phragmites longivalvis in the delta of Nak Dong River, the production, decomposition, and accumulation of organic were estimated by the theoretical analysis. The amounts of organic carbon and organic matter of litter are $1020.43g/m^2;and;591.90g/m^2$, respectively. The amounts of organic matter and organic carbon on the grassland floor is $1154.96g/m^2;and;669.93g/m^2$, the ratio of annual litter production 'L' to the amount of accumulation on the top mineral soil (F, H and $A_2$ horizons) provided estimates of decay constant k. Constant k is 0.884 in the Phragmites longivalvis community. The vertical levels of organic matter and organic carbon is the highest in 120-140 cm of photosythetic system and in 0-20 cm of non-photosyntic system.