The eutrophication in lakes is caused by the inflow of excessive nitrogen and phosphorus, which are not only pollutants to reduce the value of water resource but also nutrients for algae growth that debases water quality. Several methods have been used to judge the eutrophication grades of lakes, but the judgment results can be different with one another even under same coditions because each method is different in judgment items and their standards. A method for overcoming the problem with the judgment of eutrophication grades is, therefore, developed in this study with the application of fuzzy theory. This method allows decision makers to represent the uncertainties (differences) of results by the existing judgment methods and also incorporate associated uncertainties directly into the judgment process, so the judgment results can be made that are more realistic and consistent than those made without taking uncertainty in account.