Abstract: The shear strength is one of the important evaluation indices for the stability of soils. The investigation into the influences of the scale effects on the shear strength of coarse-grained soils has important theoretical significance and application for the strength estimation of soils in high earth-rock dam projects. According to the previous studies, the variations of the maximum particle size d_max and gradation structure can be seen as the main reasons resulting in the scale effects, and the gradation structure can be expressed by a characteristic parameter, the gradation area (S). By changing d_max or S, 24 groups of samples with different gradations of coarse-grained soils are designed. The direct shear tests on all the samples are conducted by a large-scale direct shear apparatus to quantitatively study the multiple influences of the maximum particle size and gradation structure on the shear strength of coarse-grained soils. The results show that the cohesion (c) and the internal friction angle (φ) of coarse-grained soils tend to increase with the increasing d_max, and thus a logarithmic equation relationship between c/φ and d_max is proposed. Both c and φ tend to increase with the decrease of S, and then decrease slightly after S reaches a certain level. As a consequence, an empirical relationship between c/φ and s is established based on the corresponding test results. Furthermore, a new prediction model for the shear strength of coarse-grained soils considering the scale effects is proposed, and the test results in the relevant literatures verify that the proposed model can be applied to different types of coarse-grained soils.