Abstract: The effects of loading rate and heterogeneity of meso-structure on the failure pattern and the macroscopic dynamic mechanical properties of concrete are investigated. Considering the effect of concrete heterogeneity, concrete is simulated as a two-phase composite composed of aggregate and mortar matrix at meso-scale in this work. The strain-rate effect is also accounted for and the damaged plasticity theory is employed to describe the mechanical behavior of mortar matrix. It is assumed that the aggregate phase cannot be damaged due to high strength, and thus the aggregate particles are set to be elastic. The dynamic tensile failure modes of a single-edge notched concrete specimen and an L-shaped specimen are numerically simulated. The simulation results indicate that the dynamic failure pattern and the direction of crack propagation of concrete have pronounced loading rate dependency. With the increase of loading rate, the failure mode of concrete changes from mode-I to mixed mode. A more complex meso-structure leads to a stronger interaction between the components and more complicated crack paths, and a more obvious crack branching behavior. Furthermore, as loading rate increases much more branching cracks occur within concrete and the width of the damaged region increases, implying that the fracture process at high strain rates requires more energy demand to reach failure. This should be the main reason for an increased dynamic strength of concrete.