Under　static　loadings,　concrete　has　been　demonstrated　to　exhibit　obvious　size　effect　due　to　the　heterogeneity　of　mesoscopic　composition.　Under　dynamic　loadings,　the　strain　rate　effect　has　a　significant　influence　on　the　failure　of　concrete.　It　is　therefore　of　great　importance　to　explore　the　size　effect　of　concrete　under　dynamic　loadings.　In　this　study,　the　focus　is　on　the　size　effect　in　dynamic　splitting-tensile　strength　of　concrete.　A　mesoscopic　numerical　model　for　the　simulation　of　the　splitting-tensile　failure　and　size　effect　at　different　strain　rates　(10(-5)/s　similar　to　200/s)　was　established.　The　mesoscopic　simulation　results　indicate　that　the　size　effect　on　the　dynamic　splitting-tensile　strength　of　concrete　has　an　obvious　discrepancy　with　the　static　one.　There　is　a　critical　strain　rate　in　dynamic　splitting-tensile　strength　of　concrete.　As　the　applied　strain　rate　below　the　critical　strain　rate　the　size　effect　behavior　is　restrained　and　weakened　gradually　with　the　addition　of　strain　rate.　As　the　applied　strain　rate　exceeds　the　critical　strain　rate　the　dynamic　strength　enhances　linearly　as　the　structural　size　increases.　Furthermore,　considering　the　contribution　of　strain　rate　on　the　dynamic　size　effect,　a　Static　and　Dynamic　unified　Size　Effect　Law　(i.e.　SD-SEL)　for　splitting-tensile　strength　of　concrete　was　developed.　The　proposed　SD-SEL　was　verified　by　the　numerical　results　and　the　available　test　data.