Exposing to small amount of terrestrial radiation constantly for a long period of time can be vital for people, because of possible hazards of radiation. In this context, evaluation and monitoring of terrestrial radiation, arising from the series of U-238, Th-232 and K-40, is essential. The purpose of the study is to determine the natural radioactivity level in Egirdir Lake. Egirdir Lake is one of the major water sources in Goller District and fourth largest lake in Turkey. The gamma spectrometric measurements were taken using NaI (Tl) detector. U-238, Th-232 and K-40 concentrations were measured from sediment samples collected from Egirdir Lake. In addition to that, dosimetric parameters such as outdoor gamma dose rate, annual effective dose equivalent and excess lifetime cancer risk were calculated. The mean specific activity concentrations of U-238, Th-232 and K-40 were 34.13 +/- 7.32 Bq/kg, 27.32 +/- 7.99 Bq/kg and 363.28 +/- 41.66 Bq/kg, respectively. Mean annual effective dose equivalent value of 0.058 +/- 0.007 mSv/y was comparable with the Turkey and world averages. The calculated mean excess lifetime cancer risk value of 2.18 +/- 0.25 x 10(-4) was lower than the Turkey average and nearly equal to world average value. The measured specific activity concentrations of radionuclides and the calculated results of dosimetric parameters in Egirdir Lake were lower than the Turkey average and highly comparable with world average values, reported by national and international authorities.