In　order　to　reveal　the　size　effect　meso-heterogeneity　mechanism　of　concrete　strength,　the　meso-element　equivalent　heterogeneity　model　was　established.　Tension　and　compression　tests　were　simulated　for　different　size　meso-heterogeneity　models.　The　results　showed　that　the　size　effect　of　concrete　is　rooted　in　the　meso-heterogeneity　mechanism　of　materials.　With　the　increase　of　model　size,　the　variation　coefficient　of　meso-element　elastic　modulus　increases　and　meso-heterogeneity　of　concrete　heightened.　Also,　more　low-strength　units　or　defects　were　discovered　in　the　large-size　model.　As　a　result,　the　tension　and　compression　strength　of　concrete　and　the　ultimate　strain　decreased　and　the　brittleness　of　concrete　strengthened.　Concrete　damage　region　extended　from　the　concentrated　area　to　multiple　non-penetrating　crack　zones.　©,　2015,　Tumu　Jianzhu　yu　Huanjing　Gongcheng/Journal　of　Civil,　Architectural　and　Environmental　Engineering.　All　right　reserved.