Abstract：In the past，the dependence of dynamical strength of brittle materials like rock on strain rate was studied mainly experimentally，and the theoretical aspects，especially the physical mechanism from low strain rate to high strain rate are not studied systematically. Based on available research results，the physical mechanism of strength-strain rate sensitivity in different strain rate regions and the transition between them are studied. The study result shows that in small strain rate region，strength-strain rate sensitivity is controlled by thermoactivational mechanism. With the increasing of strain rate，the macro-viscosity mechanism arises，and gradually becomes dominant. In high strain rate region，inertia effect becomes very significant，and the growth of defects of a wide range of sizes will be initiated simultaneously，and then the thermoactivational rupture of atomic bonds will take place in the area without defects. Thus the strength-strain rate sensitivity can be considered as the result of competition between the coexisting thermoactivation mechanism and the macro-viscosity mechanism，which plays dominant roles in different strain rate regions. On this basis，a unified model of competition between thermoactivational mechanism and the macro-viscosity mechanism is given，and the effectiveness of the model is verified with experiment data and good accordance is obtained with the experiment data.
