Cementitious　materials,　mainly　in　the　form　of　concrete,　belong　to　a　class　of　macroscopically　heterogeneous　materials.　The　building　technology　of　mass　concrete　structures　enables　large-sized　coarse　aggregate　to　be　used.　The　maximum　coarse　aggregate　size,　in　conjunction　with　the　mechanical　properties　of　the　interfacial　transition　zone　(ITZ),　have　great　influences　on　the　mesoscopic　crack　path　and　the　macroscopic　mechanical　behavior　of　concrete.　Herein　this　study,　based　on　some　pioneering　work　(Stroeven,　2000;　Rossello　and　Elices,　2004,　Rossello　et　al.,　2006;　Elices　and　Rocco,　2008),　the　effects　of　varying　the　maximum　coarse　aggregate　size　and　the　ITZ　strength　on　the　global　mechanical　properties　of　concrete　in　direct　tension　are　theoretically　modelled　from　a　mesoscopic　point　of　view.　The　particle　size　effect　behavior　of　concrete　is　analyzed　based　on　the　proposed　theoretical　method,　and　some　general　conclusions　can　be　drawn　as:　(1)　The　degree　of　tortuosity　of　the　mesoscopic　crack　path　in　direct　tension　increases　with　increasing　maximum　coarse　aggregate　size　and　with　decreasing　ITZ　strength;　(2)　The　fracture　energy　and　the　tensile　strength　of　concrete　increase　dramatically　with　increasing　ITZ　strength;　(3)　For　normal　strength　concrete,　the　fracture　energy　and　the　tensile　strength　decrease　with　increasing　maximum　coarse　aggregate　size,　while　for　relatively　high　strength　concrete,　the　trends　are　the　opposite;　(4)　The　properties　of　ITZ　could　significantly　affect　the　homogeneity　of　concrete.　High　quality　ITZ　could　make　the　mechanical　properties　of　concrete　more　homogeneous　and　get　a　relatively　high　strength　concrete,　consequently　a　short　characteristic　length　and　a　high　brittleness　number　are　resulted;　(5)　For　normal　strength　concrete,　the　characteristic　length　increases　with　increasing　maximum　coarse　aggregate　size,　while　for　relatively　high　strength　concrete,　it　is　fairly　insensitive　to　the　maximum　coarse　aggregate　size.　It　is　hoped　that　this　theoretical　method　may　be　helpful　in　the　study　of　size　effect　and　in　the　design　of　concrete　and　future　cementitious　materials.