Crack propagation behaviors at a mortar-aggregate interface in concrete under dynamic tensile loading conditions are investigated numerically. It is found, for a certain interfacial strength and aggregate size, that the crack can penetrate through the interface under an external load with its loading-rate higher than a threshold value. Moreover, for the crack penetration, the smaller the radius of an aggregate, the higher the loading-rate is needed. Therefore, concrete failure energy increase considerably with the loading-rate (or the strain-rate). Such a strain-rate effect on the strength of concrete is in agreement with previous experimental results.