Background: Many radioactive compounds may be released into the environment and hence into the drinking water supplies due to the human activities. Radionuclides can also enter the food chain if the contaminated water is used for drinking or irrigation purposes. Objective: To estimate the annual effective dose of natural radioactive materials like 226Ra and 40K in drinking water of wells in Zanjan, nortwestern Iran. Methods: 82% of drinking water in Zanjan comes from drilled wells. Water samples from the main sites of drinking water from different parts of Zanjan were collected. The amount of 226Ra and 40K were measured by gamma-ray spectrometry using a NaI (Tl) detector. The annual effective dose of the radioactive materials was calculated for children (2–7 years old) and adults (≥17 years old) with annual water intake of 350 and 500 L, respectively. The annual effective dose received by residents of the city due to consumption of the contaminated drinking water was also calculated. Results: The mean±SD concentrations of 226Ra and 40K were 32.4±7.8 and 20.5±27.4 Bq/L, respectively. The mean±SD annual effective dose received by residents of the city due to consumption of the contaminated drinking water was 7.13±1.76 and 4.59±1.12 mSv/y for children and adults, respectively. Conclusion: The level of radioactive compounds in water of drilled wells in Zanjan is significantly higher than the ICRP permissible limit of 1 mSv/y.