The effect of gravel contents on the cyclic and post-cyclic monotonic simple shear behavior of gravelly soils during seismic events has significant implications for civil infrastructure. Large-scale cyclic simple shear tests and post-cyclic monotonic simple shear tests were performed on gravelly soils with different levels of gravel content (GC) and loading conditions. The test results indicated that the GC of a sand-gravel mixture (SGM) profoundly influences the mixture's cyclic simple shear characteristics. The SGM specimen with the lowest GC had the broadest hysteresis loops. The shear modulus of the SGM specimens differing in GC was found to increase with increasing cyclic number. The increase in shear modulus was more pronounced during the first 10 cycles. The damping ratio of the SGM decreased to some extent as the cyclic loadings increased. Overall, the results suggested a modified relationship between the damping ratio and shear modulus. The modified function accurately represented the relationship between the normalized damping ratio and the normalized secant shear modulus for SGMs varying in GC. Shear strain obviously decreased with the number of cycles during the initial cyclic loadings, when loading frequency was low. However, at a higher loading frequency, shear strain was less affected by the number of cycles. In general, the inclusion of gravel particles slightly reduced the post-cyclic monotonic shear resistance.