Abstarct: In this research, the results of an experimental study to determine dimensions of scour hole due to free falling jets at the downstream of storage reservoirs has been presented. For this purpose, the effect of parameters such as jet discharge (Q_j), medium diameter of bed sediments (D_50) and the impinging angle of jets (θ) relative to horizontal plane were investigated. The experiments were carried out at the hydraulic laboratory of shahrood university of technology, in a rectangular flume with 1 m width, 2.43 m lenght and 1.11 m height, so that scouring was not affected by sidewalls. Water jet falls from pipes with circular section and diameter of 1.25 inch into the downstream pound. The drop height was 95 cm. The downstream pound filled with 20 cm of non-cohesive sediments with medium diameter of 1.7 mm, 3.2 mm and 6.75 mm. Experiments carried out with different discharges at the range of 1.32 to 5.14 l/s. the results of this study showed that for all experiments a scour hole performed at the downstream of falling jets and the relative scour depth (d_s/h_tw) related to particles froud number ( F_g) and impinging angle of jets (θ). The sensitive analysis of the proposed general equation showed that ±10 %, ±20 % and ±30 % changing of parameters such as Q_j, D_50 and θ, impinging angle of jets and medium diameter of the bed sediments had maximum and minimum effect to estimating d_s/h_tw, respectively. As ±30 % changing of parameters such as Q_j, D_50 and θ changes d_s/h_tw 18.65 %, 9.54 % and 57.08 %, respectively.