Abstract: The longitudinal sections of the fractured surfaces of the gabbro specimens failured by static or dynamic load are observed and analysed with SEM and optical microscope. The results show that macro-crack bifurcation is hardly found on any section for static loading, but obvious macro-crack bifurcations are found for dynamic loading. And it shows that the number and the total length of the bifuraction cracks for a specimen increase with(dynamic) loading rates. Based on the experimental results, finally, the dynamic fracture mechanism of rock is discussed.