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.　©,　2015,　Tsinghua　University.　All　right　reserved.