In the present study, the bark of oak trees (Quercus petraea Liebl.) was collected from Belgrade Forest northwest of Istanbul for determination of Cs-137, K-40, Th-232, and U-238 activity. A gamma spectrometer equipped with high-purity germanium detector was used for radioactivity measurement. Bark samples were collected from the northern and southern sides of trees at a height of 1.5 m above the ground in order to see the effects of rainfall and wind; and they were also collected from the northern side at a height of 0.5 m above the ground to assess the effect of soil splash. The activity concentrations of Cs-137, K-40, Th-232, and U-238 in the bark samples were found to vary in the range of 1.40-27.50, 45.0-221.2, 0.92-9.64, and 4.04-36.10 Bq/kg, respectively. The elevated activity of U-238 in bark samples could be attributed to a large amount of coal combustion in the region until the 1990s. According to one-way ANOVA, a significant difference was not found in bark samples collected at a height of 0.5 m and 1.5 m above the ground in terms of radionuclide activity. There was also no significant variation regarding radionuclide accumulation between northern and southern sides.