Zinc　sulfide-graphene　(ZnS-GR)　nanocomposites　with　a　high　degree　of　dispersion　and　high　coverage　of　ZnS　quantum　dots　(QDs)　have　been　synthesized　by　a　facile　solvothermal　method　without　any　dispersant,　during　which　the　formation　of　ZnS　nanoparticles　and　the　reduction　of　graphene　oxide　(GO)　occur　simultaneously.　ZnS-GR　nanocomposites　exhibit　much　higher　photoactivity　than　nanoparticle　crystal　ZnS　(NPC-ZnS)　prepared　in　the　absence　of　graphene　(GR),　as　evaluated　by　degradation　of　methylene　blue　(MB)　in　the　liquid　phase　under　ultraviolet　(UV)　light.　Among　them,　the　ZnS-GR　nanocomposite　with　a　5%　mass　fraction　of　GR　prepared　at　120°C　has　the　highest　photocatalytic　activity.　The　conversion　and　mineralization　over　MB　are　96.7%　and　57.1%　respectively,　which　is　much　higher　than　that　of　NPC-ZnS.　The　high　photoactivity　of　ZnS-GR　nanocomposites　can　be　ascribed　to　the　integrated　effect　of　an　extremely　high　specific　surface　area　and　the　excellent　electron　conductivity　of　GR　and　its　significant　influence　on　the　morphology　and　structure　of　the　samples.　Moreover,　it　is　found　that　the　oxidation　of　MB　is　driven　mainly　by　the　participation　of　ṡOH　radicals.　Accordingly,　a　potential　photocatalytic　mechanism　of　ZnS-GR　nanocomposites　in　the　photocatalytic　process　has　been　proposed　in　this　work.　It　is　expected　that　our　work　could　provide　valuable　information　on　the　design　of　metal　sulfide　decorated　GR　with　excellent　properties.　©　2013　IOP　Publishing　Ltd.