Introduction: Background radiation is originated from environmental radioactive sources cosmicandterrestrial.Materials distribution onsurface of the earth and height from sea level are not the same at various points.Absorbed dose caused by them were different at various points.In this research, Background radiation level inoutdoor different areas of Ilam province was measured and effective dose ratecaused by it was calculated. Materials & methods: RDS -110detector was used to measure background radiation. The metal base with a height of 1m was used for measuring at the same altitude and detector placed on the metal base. Outdoor backgroundradiationwas measured for seven points (The highest altitude, the lowest point North, South, East and West). Measured datawas entered to SPSS16 software for statistical operations. The mean equivalent dose rate andstandard deviation were calculated. By using the dosimetry calculations, the amount of annals effective dose rate in outdoor was calculated for different regions of Ilam province. Findings: The mean equivalent dose rate caused by outdoor background radiation for different regions of Ilam province was obtained based on unit nSvh -1 include: Ilam 84 ± 10.67, Ivan 82 ± 10.71, Dehloran78 ± 12.61, Abdanan80 ± 11.21, Sirvan97 ± 15.50, Mehran 75 ± 10.31, Malekshahi88 ± 13.78, Dareshahr95 ± 14.15, Badreh85 ± 8.82 and Chardavol93 ± 13.20.The maximum outdoor equivalent dose rate was in Sirvan area with 97 ± 15.50nSvh -1 and minimum value in Mehran area with 75 ± 10.31nSvh -1 . The average of outdoor annals equivalent dose rate in entire Ilam province 85.7± 7.5nSvh -1 in comparison average world 59nSvh -1 is more than 44%. Well as annual effective dose rate was 0.107±0.009nSvy -1 . Discussion & Conclusions: Ilam provincehasmore mean-annualequivalent dose rate and effective dose rate compared to global average annualequivalent dose rate and effective dose ratedue to mountainous and relatively high altitude