Granulated　blast　furnace　slag　(GBFS)　is　a　by-product　of　ironmaking　and　widely　used　in　the　cement　and　concrete.　GBFS　samples　with　different　ratio　of　SiO2/Al2O3　were　prepared　under　the　laboratory　conditions　and　ground　by　the　agate　ball　mill　at　different　times.　Then　they　were　characterized　by　laser　particle　size　analysis　(LPSA),　scanning　electron　microscopy　(SEM),　Bertrand　surface　method　and　mechanical　testing.　It　was　found　that　the　activity　index　of　GBFS　clearly　increased　with　the　decrease　of　the　GBFS　particle　size.　In　the　case　of　the　average　particle　size　about　32μm,　the　activity　indices　were　irregular　with　the　increase　of　SiO2/Al2O3　ratio,　whereas　for　the　average　particle　size　about　11μm,　the　activity　indices　decreased　with　the　increase　of　SiO2/Al2O3　ratio.　©　(2015)　Trans　Tech　Publications,　Switzerland.