Dams as important man-made structures need to be monitored continuously and precisely. Variations in water surface area play an important role in this major. SAR images of the Sentinel-1 mission have been considered a promising tool to monitor water dynamics due to their cloud-proof, illumination-independent, and high spatiotemporal resolution properties. In this study, to monitor the surface area of the Doroudzan dam reservoir, SAR images of Sentinel-1A mission in two polarization (VV and VH) in 2018 and 2019 are used. After image pre-processing, images are classified into two-class, i.e. water and non-water, based on the thresholding method, and corresponding threshold values are selected from the image. Next, the area of the water body was computed. Then, the time series of the surface area of the dam reservoir is obtained from VV and VH polarizations and was compared with the time series obtained from in-situ areas computation of the Doroudzan dam reservoir. The results showed that: (1) The threshold values for the classification of images and water separation from non-water for VH from -21.36 to -23.01 dB and VV from -13.47 to -19.08 dB. (2) VV polarization with relative RMSE of %5.83 and correlation coefficient of %97.55 compared to in-situ surface areas achieved higher accuracy as compared to VH polarization which resulted in relative RMSE value of %9.21 and correlation coefficient of %83.63 as compared to in-situ areas; VV is more sensitive to water cover than VH and is more stable under seasonal variations than VH. (3) The surface area of the Doroudzan dam reservoir area had started to increase in February 2018, and then declined in May 2018; It also started to rise again in December 2018 and then declined in May 2019. The surface area obtained from VV polarization of the Doroudzan dam reservoir was at its maximum on 20 April 2019 with 43.6187 km2 and a minimum on 28 September 2018 with 24.2241 km2.