Great　progresses　have　been　made　in　static　size　effect　of　concretes,　while　few　efforts　have　been　conducted　in　dynamic　size　effect　of　concretes.　Herein　the　study,　in　order　to　explore　the　size　effect　of　concrete　under　dynamic　loadings,　considering　concrete　heterogeneities　and　the　strain　rate　effect　for　the　meso　components,　a　meso-scale　simulation　method　was　built.　Taking　the　double　notched　concrete　specimens　as　examples,　the　dynamic　tensile　failure　and　the　size　effect　of　concrete　under　low　strain　rates　from　10-5　s-1　to　1　s-1　were　investigated　using　the　meso-scale　simulation　method.　The　effect　of　strain　rate　on　the　size　effect　in　tensile　strength　of　concrete　was　also　analyzed.　Finally,　based　on　the　influencing　mechanism　of　strain　rate　effect　on　dynamic　strength　and　size　effect,　i.e.　the　enhancement　of　strength　and　weakening　of　size　effect,　a　＇static-dynamic　unified　size　effect　law＇　for　dynamic　tensile　strength　of　concrete　was　established.　The　proposed　size　effect　law　was　also　calibrated　by　meso-scale　simulation　results.　©　2019,　Engineering　Mechanics　Press.　All　right　reserved.