Abstract: The geometrically similar cracked straight through flattened Brazilian disc specimens of different sizes (80, 122, 155mm) were diametrically impacted by the split Hopkinson pressure bar system, and the dynamic fracture toughness of rock materials was determined by the experimental-numerical method. The result shows that the dynamic fracture toughness is affected by both the loading rate and the specimen size. In order to separate the coupling effects of these two factors on the dynamic fracture toughness, two procedures of linear regressions were conducted: the first regression was used to treat the dynamic fracture toughness with respect to the loading rate for the same sized specimens, and by using the obtained formulas, the dynamic fracture toughness corresponding to different diameters under the same loading rate can be calculated, under this condition the Bazant’s static size effect law can be applied to dynamic situation; then the second regression was made for the dynamic fracture toughness with respect to the specimen size. The size effect law of dynamic fracture toughness under a certain loading rate was thusly obtained. Finally, the dynamic fracture toughness of the marble material under different loading rates was presented.