We have developed a three dimensional water quality .modeling system that consists of 3-D hydrodynamic model ROMS and eutrophication model CE-QUAL-ICM. Two models are directly coupled with linkage module that exchanges information calculated in structured and unstructured grid systems. The new modeling system ROMS-ICM has been set up for the Saemangeum tidal lake and verified successfully with long-term observed data on water quality state variables. Also we anticipate the applicability of the ROMS-ICM to modeling the water quality and possible extension to ecosystem in Yellow Sea.