The　macro　nonlinearity　and　size　effect　of　concrete　are　essentially　attributed　to　the　internal　heterogeneity　of　concrete.　Considering　the　heterogeneities　and　the　strain　rate　effect　of　meso-components,　the　concrete　was　modelled　as　a　three-phase　composite　composed　of　aggregate　particles,　mortar　matrix　and　the　interfacial　transition　zones　(ITZs).　Taking　double-edge　notched　concrete　specimens　as　examples,　a　meso-scale　numerical　model　for　the　simulation　of　the　failure　and　size　effect　of　concrete　under　dynamic　tensile　loading　with　different　strain　rates　from　10(-5)/s　to　100/s　was　established.　The　effect　of　structural　sizes　on　the　dynamic　tensile　strength　of　concrete　was　explored.　The　numerical　results　indicate　that　dynamic　size　effect　on　tensile　strength　of　concrete　has　an　obvious　discrepancy　with　the　static　one.　There　exists　a　critical　strain　rate　((epsilon)　over　dot(cr)　=　1/s),　and　at　the　strain　rate　the　dynamic　tensile　strength　turns　to　be　independent　on　the　structural　size　of　the　specimen.　For　the　case　less　than　the　critical　strain　rate,　the　tensile　strength　decreases　with　increasing　the　specimen　size,　and　the　size　effect　on　the　dynamic　tensile　strength　is　weakened　or　suppressed　as　the　strain　rate　increases.　For　the　strain　rate　larger　than　the　critical　one,　there　is　no　obvious　difference　in　the　dynamic　tensile　strength　of　the　concrete　specimen　having　different　structural　sizes,　and　the　corresponding　size　effect　completely　disappears.　According　to　the　influencing　mechanism　of　strain　rate　effect　and　size　effect,　a　Static　and　Dynamic　unified　Size　Effect　Law　(i.e.　SD-SEL)　for　tensile　strength　of　concrete　was　built.　The　proposed　SD-SEL　was　also　verified　by　the　mesoscopic　numerical　simulation　results.