Besides the assessment of radionuclide accumulation in marine sediment along the Upper Gulf of Thailand by HPGe-gamma spectrometry, the natural radionuclides including U-238,Th 232 and K-40 were found with mean specific activities of 39 ± 6.41 ±7 and 472 ± 81 Bq/kg in summer ; 44± l0.59± 17 and 463± 94 Bq/kg in rainy season. and 4 l ±6. 50 ± 9 and 484 ±83 Bq/kg in winter, respectively No contamination of the artificial radionuclide (Cs-137) was found. Accumulation of radionuclides marine sediments depended on chemical properties of radionuclides, physical and chemical properties of the sediments: type of sediments , grain size and organic content, and geography: season, stream direction and flow rate. The radionuclide levels in marine sediments were higher than the global mean values reported by UNSCEAR. However, the radiation hazards associated with the radioactivity in the sediments were not expected.