Control and estimation of the first impounding and long-term seepage behavior of dams is of great importance. One of the most common methods to reduce the amount of seepage through the foundation of embankment dams is using a concrete cut-off wall. In this paper, effectiveness of the plastic-concrete cut-off wall of one of the recently completed and impounded embankment dams in Iran, namely Silveh Dam, with a height of 77 meters is evaluated by three-dimensional modelling of the whole body, foundation, abutments, and the reservoir. In this regard, 796numerical modelling and analysis of this dam with and without cut-off wall have been performed, and the amount of outflow discharge and hydraulic gradients within the foundation are compared in three sections of the dam. It should be noted that the results of the analysis of the model having the cut-off wall are verified by the instrument measurements in a specific reservoir water level. Results reveal that the presence of the cut-off wall decreases the amount of outflow discharge for at least 11%, and reduces the hydraulic gradients in the dam foundation for 33, 66, and 71% in the studied sections. As a result, the long-term water barrier function of the plastic-concrete cut-off wall when the reservoir level will reach to NWL. next year, is predicted to be completely acceptable considering the reduction of the amount of outflow discharge and hydraulic gradients, which results in improving the stability of the dam body and its foundation.