The purpose of this work is to integrate GIS techniques into the eutrophication assessment process of a large scale coastal area – Bohai Bay. Specifically, based on ArcGIS8.3 Desktop, a geodatabase was built for the management of the field monitoring data of Bohai Bay. Using the existing functions of the GIS platform, NQI index and a fuzzy assessment method were adopted respectively for the assessment of the trophic state of this area. Both methods generated salient spatial variations in the assessment results which were often missed by other attempts and thus demonstrated the power of the GIS techniques. The specific trends of the results provide clues for further studies.