The　particle　size　of　coarse　aggregates　in　dam　concrete　ranges　from　5mm　to　150mm,　consequently,　the　macroscopic　mechanical　properties　of　concrete　are　easily　varied　with　the　aggregate　gradation.　From　the　mesoscopic　of　view,　a　theoretical　method　to　predict　the　mechanical　properties　of　concrete　considering　of　the　size　effect　in　material　is　established.　In　view　of　the　internal　structures,　the　concrete　material　is　regarded　as　a　three-phase　composite　composed　by　the　coarse　aggregate,　the　mortar　matrix　and　the　interfacial　transition　zone　(ITZ).　It　is　assumed　that　the　aggregate　is　unbroken　within　the　process　of　loading.　The　mechanical　behavior　of　the　mortar　matrix　and　the　ITZ　is　described　by　bilinear　constitutive　models.　Based　on　the　theoretical　concept　of　fracture　mechanics,　a　group　of　theoretical　formulae　for　the　determination　of　mechanical　parameters　of　concrete　including　the　mesoscopic　crack　length,　the　macroscopic　fracture　energy,　the　uniaxial　tensile　strength,　the　characteristic　length　and　the　brittleness　number　are　derived.　The　influences　of　the　mechanical　properties　of　the　ITZ　and　the　aggregate　gradation/volume　on　the　macroscopic　mechanical　properties　of　concrete　are　analyzed　based　on　the　proposed　theoretical　method.　©　2018,　China　Water　Power　Press.　All　right　reserved.