Soil is one of the main national wealth of each country, which the serious planning must be taken to preserve it. Growing populations and foods demand in the recent years, have led farmers to exploit marginalized lands such as forests and pastures in different countries. This is while marginal lands with high erosion potential have low production potential. Over the past few decades, surface erosion has caused many obvious and hidden problems that need research and scientific planning in order to prevent this destructive process. Researchers from all over the world have been seeking from past times to calculate erosion rates in different lands by using different methods. The use of radioactive materials is a quick and economical tool for describing soil erosion. Cesium-137 is a synthetic radioactive element with a half-life of 30.2 years, which wasreleasedto the earth atmosphere due to nuclear tests and explosions with rainfall in the 1950s-1970s. Fallen cesium-137 is highly adsorbed on clay and organic matter and is concentrated in surface horizons. Physical processes such as erosion and tillage are the main reason for the redistribution of cesium-137 in the soil. Therefore, by measuring the amount of cesium-137 in soil samples, annual soil erosion can be calculated. This research was carried out with the main purpose of measuring the activity of cesium-137 and soil erosion in soil samples collected from two hill slopes with pasture and agricultural application in Semirom_ south of Isfahan province. Each hill has five different positioning slopes, including:summit, shoulder, back slope, foot slope andtoe slope, and in each position, two depths of 0-15 and 15-30 cm were sampled by 20 to 20 cm cross-sectional sampling.High purity germanium detector (HPGe), a multichannel analyzer, and gamma vision software were used to determine the radioactive elements of 20soil samples. By using the cesium-137 method, the average rate of soil erosion in agricultural application samples was found to be 105.62 ton per hectare per year, and 63.33 ton per hectare per year in pasture application samples, both of which are higher than the mean rate of natural soil erosion 20 tons per hectare per year due to wind, rain and flood. Therefore, it is necessary to consider measures such preventing of further agricultural use and grazing livestock in these areas. Also, in this study, the concentrations of natural radionuclides such as uranium-238, thorium-232, potassium-40 and radium-226 in soil samples were measured. According to the measurements, it was found that the concentration of the above mentioned radioactive elements were in the range of 13.63 to 29.29, 14.11 to28.68, 255.14 to 639.22 and 44.92 to 79.49 Becquerel per kilogram respectively,that all of these values ??are within natural limits.