Abstract: The macroscopic resistance of concrete under dynamic loading includes two parts. One is the actual dynamic strength of concrete and the other is the additional resistance contributed by inertia effect. Most existing researches on the dynamic strength of concrete do not distinguish the actual dynamic strength and the inertia resistance. Thus, they would overestimate the actual dynamic strength of concrete in actual projects. In fact, the actual dynamic strength of concrete will tend to a certain value when the strain rate reaches a very high value, which is the ultimate dynamic strength of concrete. This paper studies the physical mechanisms governing the strain rate-dependent behavior of concrete strength. The macroscopic resistance of concrete is divided into the actual dynamic strength and the additional resistance caused by inertial force. The physical mechanisms governing their mechanical properties are analyzed. The mechanisms indicate that the actual dynamic strength of concrete dose not increase infinitely but has an ultimate dynamic strength. This strength rule is the same as the rule of the nonlinear dynamic uniaxial S criterion for concrete. Therefore, the S strength criterion is developed to describe the actual dynamic strength of concrete. Dynamic uniaxial compressive test results and dynamic uniaxial tensile test results are used to verify the criterion. The comparisons indicate that the S criterion is capable of describing the actual dynamic strength and reflect the ultimate strength at a high strain rate of concrete.