Land Surface Temperature (LST) is an important indicator of the study of energy balance models at the earth's surface and the interactions between the Earth and the atmosphere on a regional and global scale.In the last few decades, different algorithms have been developed to determine the land surface temperature using various satellite images. In this study, a new split window method for estimating the land surface temperature using Sentinel-3A Satellite Radiometer (SLSTR) images is presented. The advantage of the proposed method is the introduction of atmospheric water vapor into the split window algorithm, plays an important role in estimating the LST. The LST was calculated by using the proposed method and the split window three algorithms. . Then the results of the proposed method and three split window algorithms were compared with the ASTER, MODIS and Sentinel-3 LST products. The RMSE values of the proposed method for the study area of East Tehran were 3.49, 1.22 and 1.26 K, respectively, compared to the ASTER, MODIS and Sentinel-3 LST products which is lower than the RMSE obtained from other split window algorithms. Also the proposed method and three split window algorithms were implemented for northwest of Isfahan and Kermanshah. For Kermanshah study area the results were much better than the other two cases and the other methods so the RMSE of the proposed method was calculated to be 1.05 Kelvin while the RMSE of the other methods was 1. 19, 1.28 and 1.56 Kelvin.