As the earth's temperature rose, the occurring of algae increased. Subsequently, a large number of bacteria grew from algae and cyanobacteria can produce geosmin and 2-MIB. Geosmin is similar to earthy taste when it rains, and human sense of smell is very sensitive to geosmin and starts to smell from 5 ppt. Therefore, in this study, a field-effect transistor using graphene, geosmin was successfully detected with a high sensitivity of 0.01- 0.1 nM in the sample solution. Our approach can be applied to detect harmful substances in various water sources.