Abstract：A diametrical impact experiment with three loading angles(the angles between loading direction and crack line are 0°，10° and 19°respectively) was performed for the geometrically similar cracked straight through flattened Brazilian disc specimens of three sizes(f50 mm，f130 mm，f200 mm) by split Hopkinson pressure bar system. Time-history curves of stress intensity factors(KI(t) and KII(t)) of mixed-mode I-II dynamic fracture，as well as the mode mixture ratio(KI(tf)/KII(tf)) at the initial time of fracture，were determined by experimental numerical method. Within loading time of 100 μs，the mode mixture ratio is affected by the size of specimen：when loading angle is 10°，in which the static condition is mixed-mode loading，the specimen of f50 mm is still in the state of mixed-mode loading，and so is under the dynamic loading condition；however，the f130 mm and f200 mm specimens are in the state of mode II loading. The difference is caused by the disturbance of interaction of stress wave and crack surfaces. Rock fracture toughness of both modes I and II(KId and KIId) has size effect；and the size effect of KIId is much more remarkable than that of KId，which can be explained from the viewpoint of energy. Considering the first nonsingular constant term in Williams series expansion of stresses near the crack tip，T stress has no effect on the computation of dynamic stress intensity factor，however it affects the crack initiation angle for the mixed-mode dynamic fracture.
