The activity concentrations of Ra-226, Th-232, and K-40 are measured in soil samples from various locations in the Kadikoy and Uskudar district of Istanbul (Turkey). The Ra-226 activity concentrations range from 19.97Bqkg(-1) to 50.80Bqkg(-1) and average Ra-226 concentration value 31.40Bqkg(-1) was calculated. The Th-232 activity concentrations range from 21.38Bqkg(-1) to 52.61Bqkg(-1) and average Th-232 concentration value 34.44Bqkg(-1) was calculated. The K-40 activity concentrations range from 464.06Bqkg(-1) to 711.27Bqkg(-1) and average K-40 concentration value 619.59Bqkg(-1) was calculated. In addition, radium equivalent (Ra-eq), absorbed gamma dose rate (D), annual effective dose equivalent, (AEDE), excess lifetime cancer risk (ELCR) were calculated in this study. All of the calculations have been compared with both national and international standards and similar studies. As a result of this comparison, levels of natural radioactivity and radiological effects were slightly higher than the World average and Turkey.