A　comprehensive　finite　element　analysis　at　the　mesoscopic　level　has　been　conducted　into　the　complex　topic　of　size　effect　coupling　dynamic　strain-rates.　Taking　the　lightweight　aggregate　concrete　(LWAC)　dumbbell-shaped　samples　as　the　object　of　numerical　investigation,　the　influence　of　strain-rate　(with　the　range　of　10(-5)/s　similar　to　100/s)　on　direct-tensile　failure　of　LWAC　(including　different　lightweight　aggregate　volume　fractions　V-agg　=　40%,　30%　and　20%)　was　discussed.　Subsequently,　the　structure　size　of　LWAC　samples　was　further　expanded　(width　W　=　100,　200　and　300　mm)　and　the　dynamic　size　effect　on　direct-tensile　strength　was　investigated.　Numerical　results　show　that　both　the　direct-tensile　strength　and　its　corresponding　size　effect　of　LWAC　exhibit　a　strain-rate　dependent　behaviour.　The　increasing　strain-rate　can　gradually　weaken　the　size　effect　of　LWAC　and　direct-tensile　strength　would　be　independent　to　the　structure　size　as　the　strain-rate　reaches　the　critical　strain-rate.　The　increasing　lightweight　aggregate　volume　fraction　can　reduce　direct-tensile　strength.　Furthermore,　a　dynamic　size　effect　model　establishing　the　direct　link　between　the　strain-rate　effect　and　size　effect　was　proposed,　which　can　quantitatively　predict　the　dynamic　direct-tensile　strength　of　LWAC.