The　size　effect　of　concrete　in　material-level　is　assumed　to　be　attributed　to　the　uncertainty　of　properties　and　scales　of　meso　structures,　including　the　coarse　aggregates,　the　cement　mortar　and　the　interfaces.　Firstly,　the　prediction　method　for　the　global　mechanical　properties　of　different　graded　concrete　is　modified　through　theoretical　analysis,　based　on　the　established　concrete　mode-I　fracture　model　that　can　take　the　meso　structures　into　consideration.　Secondly,　an　analysis　theory　for　size　effect　of　concrete　in　material-level　is　proposed　and　the　variation　of　the　global　mechanical　properties　of　concrete　with　properties　and　scales　of　meso　structures　is　analyzed.　The　analysis　shows　that,　the　interface　properties,　the　aggregate　volume　and　the　maximum　aggregate　size　can　significantly　influence　the　percentage　of　interface　cracks　within　the　mesoscopic　crack　of　concrete　with　strong　aggregates　under　uniaxial　tensile　load,　and,　consequently　the　variation　of　the　global　mechanical　properties　of　concrete　with　properties　and　scales　of　meso　structures.　©　2019,　China　Water　Power　Press.　All　right　reserved.