The Sandikli basin is located in the southwestern part of Turkey. In this paper, groundwater stable isotope analyzes were evaluated to understand the recharge of the aquifer system in the Sandikli basin. For this, samples of groundwater were collected for two periods in 20092010 years and the stable isotope and natural radioactivity analyzes were made in the basin. The delta O-18 and delta H-2 contents of groundwaters in the region range from -8.08 to -10.77% and from-52.44 to -69.53%, respectively. The stable isotope data lie generally above the Global Meteoric Water Line and waters are meteoric origin. The tritium content of the groundwaters ranges from 0.00 to 4.20 TU in dry period and from 0.30 to 5.40 TU in rainy period, respectively. These delta H-3 values of waters could be represented that this waters are deep aquifer waters and slow circulation. The determination of the natural radioactivity in groundwater is important in terms of human health. The gross alpha and gross beta radioactivity concentrations of waters varied between 29-162 mBq L-1 and 70-330 mBq L-1, respectively. The results have shown that gross a and beta radioactivity concentrations in drinking waters in the basin are relatively low and suitable the World Health Organization regulations for drinking water.