Abstract: Considering the meso-cale heterogeneity, concrete was deemed as a four-phase composite composed of aggregate, mortar matrix, interface transitional zone (ITZ) and the initial defect in this paper. A two-dimensional random aggregate model of concrete beam was established. The plastic damaged model combined with the material strain-rate effect was employed to describe the mechanical properties of mortar matrix and the ITZ. It is also assumed that the aggregate was elastic, which would not be damaged. The numerical flexural-tensile study of the concrete beams consists of three different models, i.e. non-initial defects model, 2% porosity model, and 5% porosity model. The influences of initial defects and loading-rates on the flexural-tensile failure mode, flexural-tensile strength and macroscopic stress-strain relationship were discussed. The numerical results indicate that the flexural-tensile failure mode of concrete and the macro mechanical properties have obvious loading-rate dependency, while the existence of initial defect has great influence on concrete failure mode and macroscopic mechanical properties.