Abstract: In the experiments, two types of holed-cracked flattened Brazilian disc specimens were adopted, one was geometrically similar with the diameters of 42, 80, 122 mm and 155 mm respectively, the other was a one-size specimen geometry with an identical 80-mm diameter and varying crack lengths. The discs were diametrically impacted by the split Hopkinson pressure bar, and the strain waves and typical failure patterns of the specimens were given. It is concluded that the rock dynamic fracture toughness increases with the increasing diameter size for the geometrically similar specimens; however, it increases and then decreases with the increasing crack lengths for the specimens of the identical 80-mm diameter. The size effect of rock dynamic fracture toughness is caused by the fracture process zone length l and incubation time . To reduce the size effect, a method to determine the rock dynamic fracture toughness was proposed by averaging the integration of dynamic stress intensity factor distribution in the spatio-temporal domain.