The eco-hydrodynamic model was used to estimate the primary productivity of the oyster culture grounds in Kamak Bay. It is composed of the three-dimensional hydrodynamic model for the simulation of water flow and ecosystem model for the simulation of phytoplankton. The ecosystem model was applied to simulate phytoplankton biomass during culturing period in condition of no oyster culture grounds. The field surveys were conducted from May, 1994 to March, 1995 in Kamak bay. The results showed the concentration of chlorophyll $alpha$ to be $1.00~23.28;{mu}g/l$ in the surface layer, $1.27~29.97;{mu}g/l$ in the middle layer and $1.23~23.08;{mu}g/l$ the bottom layer. In monthly variations of chlorophyll $alpha$ concentration, very high concentration were found in July, 1994 and very low concentrations in December, 1994. As the results of three-dimensional hydrodynamic simulation, the computed tidal currents ave mainly toward the inner part of bay through Yeosu Harbor and the southern mouth of a bay during the flood tide. The computed residual currents were dominated southward in Yeosu Harbor and eastward in the mouth of bay and also showed strong clockwise water circulation at the mouth of bay. The pattern between the simulated and observed tidal ellipses at three stations was very similar. The mean relative errors of all levels between the simulated and observed phytoplankton biomass at 14 stations in Kamak Bay were $13.81\%,;9.31\%;and;17.84\%$, respectively. The results of phytoplankton biomass simulation showed that the biomass increased from June to September and rapidly decreased to December and then slowly increased to March. Primary productivity was estimated in the range of $0.99~10.20gC/m^2/d$ with the average value of $4.43gC/m^2/d$ in condition of no oyster culture grounds. Primary productivity was rapidly increased from lune to August and rapidly decreased to December and then slowly increased from January to March in Kamak Bay.