Diatoms are commonly used as bioindicator organisms for evaluating especially freshwater quality. Trophic Diaton Index (TDI) provides information about trophic levels of the aquatic ecosystem. In this study, the epipelic diatoms were collected from Porsuk Dam Lake in autumn season of 2009. As a result, total of 25 diatom taxa were identified and the water quality of Porsuk Dam Lake in terms of trophic levels was determined according to these 25 taxa by calculating TDI. It was determined that, Stephanodiscus agassizensis Hakansson & Kling, Epithemia sorex Kützing and Aulacoseira granulata (Ehrenberg) Simonsen were the most dominant taxa and Porsuk Dam Lake has mesotrophic state according to TDI