The rate of soil erosion and sediment yield estimation in the catchment areas is considered as an important factor for soil and water conservation practices. Due to difficulties and restrictions in common soil erosion methods, applying a high accuracy method with lowest cost and time consuming would be necessary. Assessment of soil loss in hilly areas with capacity of rang land that has changed to cultivation areas because of high population and evaluation of Cesium 137 method to estimate soil loss constitutes two main purposes of this research. So, the Cesium 137 method was performed to measure soil loss in Raymaleh watershed where land use alteration is common. To do so, soil sampling and measuring soil surface features were done in seventeen transects composed of eleven transects on hilly area (complex slopes with 5-50 percent) and six transects on downhill areas (simple slopes with 2-8 percent) were performed by association with aerial photographs (1:20000) and topography map (1:50000). In addition, for complex slopes (including upper, shoulder, back, foot and top parts) and simple slopes (including upper, middle and lower parts) according to slope length between 2 to 7 soil samples (0-27 cm) the taken and Cesium-137 measured by Spectrometer device. Results showed that the average soil loss in different parts of complex slope including summit, shoulder and back slope respectively was 31.74, 66.28 and 48.77 ton/ha/yr, whereas for simple slopes the losses 78.60, 54.47, 79.54 ton ha^-1 yr^-1. The rate of sedimentation in non erosion parts such as top slope was 64.27 ton ha^-1 yr^-1 and for foot slope soil deposition was more dominant than soil loss. Generally, we may conclude that Cesium 137 could be considered as a high accuracy and fast method to estimate soil loss and sedimentation.