The　macroscopic　resistance　of　concrete　under　dynamic　loading　includes　two　parts.　One　is　the　actual　dynamic　strength　of　concrete　and　the　other　is　the　additional　resistance　contributed　by　inertia　effect.　Most　existing　researches　on　the　dynamic　strength　of　concrete　do　not　distinguish　the　actual　dynamic　strength　and　the　inertia　resistance.　Thus,　they　would　overestimate　the　actual　dynamic　strength　of　concrete　in　actual　projects.　In　fact,　the　actual　dynamic　strength　of　concrete　will　tend　to　a　certain　value　when　the　strain　rate　reaches　a　very　high　value,　which　is　the　ultimate　dynamic　strength　of　concrete.　This　paper　studies　the　physical　mechanisms　governing　the　strain　rate-dependent　behavior　of　concrete　strength.　The　macroscopic　resistance　of　concrete　is　divided　into　the　actual　dynamic　strength　and　the　additional　resistance　caused　by　inertial　force.　The　physical　mechanisms　governing　their　mechanical　properties　are　analyzed.　The　mechanisms　indicate　that　the　actual　dynamic　strength　of　concrete　dose　not　increase　infinitely　but　has　an　ultimate　dynamic　strength.　This　strength　rule　is　the　same　as　the　rule　of　the　nonlinear　dynamic　uniaxial　S　criterion　for　concrete.　Therefore,　the　S　strength　criterion　is　developed　to　describe　the　actual　dynamic　strength　of　concrete.　Dynamic　uniaxial　compressive　test　results　and　dynamic　uniaxial　tensile　test　results　are　used　to　verify　the　criterion.　The　comparisons　indicate　that　the　S　criterion　is　capable　of　describing　the　actual　dynamic　strength　and　reflect　the　ultimate　strength　at　a　high　strain　rate　of　concrete.　©　2018,　Engineering　Mechanics　Press.　All　right　reserved.