Gabions are water structures that consist of metal mesh boxes contain rocks. This makes gabions very economic and simple. Those structures are widely used in small head works to dissipate the potential energy, and decrease scour. This study aims to investigate the effect of particle grain size on the hydraulic performance and its impact on scour downstream gabion stepped spillway to provide the designers with a wide range of available diameters for use in construction. For this purpose, six experimental models of gabion stepped spillway were tested. The used stepped gabion spillway model has 5 steps with 20 cm height, the steps are 7cm horizontal and 4 cm vertical each. The gabions models of different single particles diameters (5mm, 10mm, 14mm, 20mm and 30mm) and solid spillway model were tested. Also six additional models of two mixtures have been tested, the first with particles diameter 10 mm and 30 mm, the other with diameter 5 mm and 20 mm, they have been mixed in three different volume ratios (1:1, 1:2, 1:3), the equivalent diameter was calculated for each mixture. The results indicated that the increase in relative diameter in single size and mixed models increases energy dissipation and decreases scour. The model with relative diameter 0.15 gave the best results. It causes reduction of Y2/Yup, Ds/Yup by about 9.69%, and 40.19 % respectively and increase in ∆E/Eup by 11.7% compared to the solid gabion model. Finally, the results of this study may be recommended in the field of applications.