In　this　research,　the　influence　of　width　on　mechanical　performance　of　joints　is　studied　by　numerical　tests　based　on　2D　mesoscale　models.　Meanwhile,　the　effects　of　concrete　strength　and　aggregate　particle　size　on　size　effect　of　joints　are　analyzed.　Firstly,　the　proposed　2D　mesoscale　model,　which　is　reflective　of　the　heterogeneity　of　concrete　and　damage　development　within　joints,　is　validated　based　on　existing　experimental　results.　Secondly,　based　on　the　proposed　2D　mesoscale　model,　a　range　of　numerical　tests　about　size　effect　are　conducted.　It　turns　out　that,　the　size　effect　of　shear　capacity　becomes　more　remarkable　with　the　increasing　concrete　strength　and　the　decreasing　aggregate　size.　Thirdly,　the　size　effect　of　joints　is　discussed　based　on　different　SELs　(size　effect　laws).　The　Type-II　SEL　proposed　by　Bazant　(1984)　is　initially　adopted　to　evaluate　the　size　effect　of　joints.　Although　it　can　give　generally　adequate　trends　on　size　effect,　the　nominal　shear　strength　of　joints　considering　the　effect　of　concrete　strength　and　aggregate　size　cannot　be　calculated　quantitatively.　Therefore,　the　authors　present　a　new　SEL　that　considers　concrete　strength　and　aggregate　size,　which　is　able　to　give　concise　shear　capacity　of　joints.