Abstract: It is known that grain sizes has a significant impact on the dynamic mechanical characteristics of the rock.In order to investigate the effect of grain size on the dynamic splitting mechanical properties and the fracture surface roughness of granite,dynamic splitting tests of different strain rates were carried out on coarse- and fine-grained granite samples.The results show that the dynamic splitting strength and maximum absorbed energy of the two types of granite samples increase with the increase of strain rate,showing a significant strain rate effect.The grain size has no significant effect on the dynamic splitting strength and the maximum absorbed energy under similar strain rate conditions.The energy efficiency of coarse- and fine-grained granite samples decreases with the increase of the strain rate with different decreasing trends.There exists a threshold strain rate for coarse-grained granites,and the energy efficiency decreases significantly under average strain rates less than 35.24s^-1,while decreases slowly for strain rates greater than 35.24s^-1.The final fracture mode of the coarse-grained granite is the type-Ⅱ splitting,and the end wedge damage zone develops towards the center with the increase of strain rate; while the final fracture mode of the fine-grained granite changes from type-Ⅰ splitting to type-Ⅱ splitting with the increase of the strain rate.Quantitative analysis of the roughness of the coarse- and fine-granite fracture surface based on the 3D section scanning data shows that the section roughness indices Z₂ and R_s of coarse-grained granite are generally greater than those of the fine-grained granite under the same strain rate,and the dynamic splitting parameters predicted based on the section roughness index Z₂ is superior to R_s.