In this research paper,  we have studied the effect of using an impermeable blanket  and compare it with the usage of cutoffs on the uplift pressure under concrete dams. This comparison has been conducted numerically, by using the element method, with the help of the Groundwater Modeling System software program. The length of the impermeable blanket  has been modified according to length of the dam base, for different values of length of cutoffs. It has been found that the existence an impermeable blanket with two cutoffs doesn't lead to decreasing the uplift pressure force in comparison with the state of the non-existence of an impermeable blanket. Also, it has been concluded that using an impermeable blanket with one single cutoff at the beginning of the dam leads to decreasing the uplift pressure force by 17% in comparison with the state of the non-existence of the an impermeable blanket, if the length  of blanket is larger than the depth of the front cutoff.