Abstarct: A number of parameters must be calculated in design of Watertight System of Dams in order to prevent water seepage. The optimum depth and the optimum distances between boreholes are the foremost parameter in their design. These parameters usually selected in accordance to sealing criteria which is baxsed on Lugeon permeability test results. Being experimental relations, these criteria could not be useful for all type dams. As a consequence, the effect of variation of depth and other parameters of the watertight system on amount of water seepage in foundations of dams must be separately taken into consideration. For assessing of these parameters in the case of Talwar dam, primarily, permeability and ability of grouting for bedrocks in the both left and right embankments and foundation including their joints and gaps in the different depths sections have been studied by applying boreholes data of the region. Thereafter, grouting pattern of watertight system in bedrocks of dam axis has been presented and evaluated by utilizing Rock Quality Designation (RQD) Secondary Permeability Index (SPI), amount of grout specific absorption, and appropriate classification. Results demonstrate that for preventing leaking water in foundation, the watertight system should have four grouting borehole sets by maximum 60 or 70 meter depth, and the grouting must be done by different mixing percentages. Subsequently, by applying special 2D numerical and analytical methods in discontinuous regions, geological data and structural analysis of existing joint sets, structural model and foundation of Talwar dam have been designed with UDEC software regarding boundary conditions such as water pressure behind the dam, and then, the amount of discharge beneath the foundation has been calculated. In this manner, in addition to estimating of seepage beneath the foundation, the appropriate Grouting Depth has been evaluated by using data of numerical and analytical results, concluding optimum depth of watertight system between 50 and 60 meter. Finally, the appropriate Grouting Pattern and other principle parameters in design of Watertight System have been studied considering discontinuity conditions and boreholes results.