In this study, uranium and thorium concentrations in the sediment samples collected from the Gulf of Izmit in Marmara Sea were determined using neutron radiography method and alpha-autoradiography method. Mean uranium concentrations calculated for the sediment samples collected from the boreholes S2, S3, S5, S8 and KS-2 were found to be 5.99 mu g/g (3.58-8.16 mu g/g), 4,59 mu g/g (2.54-8.22 mu g/g), 4.03 mu g/g (2.13-6.9 mu g/g), 3.71 mu g/g (2.36-4.48 mu g/g) and 2.81 mu g/g (2.16-4.06 mu g/g), respectively. The highest uranium concentration in all borehole samples was obtained from borehole S3 with a value of 8.22 mu g/g at a depth of 77.00 m, followed by borehole S2 with a value of 8.16 mu g/g at 76.95 m depth. These results also confirms the possibility of thermal sources at boreholes S2 and S3. The mean thorium concentrations in boreholes S3 and S8 were found to be 15.26 mu g/g (9.60-17.49 mu g/g) and 15.76 mu g/g (13.89-17.37 mu g/g), respectively. The highest thorium concentration in obtained samples from all boreholes was obtained in borehole S3 with a value of 17.49 mu g/g at a depth of 77.00 m.