Water-Soluble,　single-crystalline,　and　amine-functionalized　graphene　quantum　dots　(GQDs)　with　absorption　edge　at　similar　to　490　nm　were　synthesized　by　a　molecular　fusion　method,　and　stably　deposited　onto　anatase　TiO2　nanoparticles　under　hydrothermal　conditions.　The　effective　incorporation　of　the　GQDs　extends　the　light　absorption　of　the　TiO2　nanoparticles　from　UV　to　a　wide　visible　region.　Moreover,　amine-functionalized　GQD-TiO2　heterojunctions　can　absorb　more　02　than　pure　TiO2,　which　can　generate　more　center　dot　O-2　species　for　MO　degradation.　Accordingly,　the　heterojunctions　exhibit　much　higher　photocatalytic　performance　for　degrading　methyl　orange　(MO)　under　visible-light　irradiation　than　TiO2　alone.　At　optimum　GQD　content　(1.0　wt　%),　an　apparent　MO　decomposition　rate　constant　is　15　times　higher　than　that　of　TiO2　alone,　and　photocurrent　intensity　in　response　to　visible-light　excitation　increases　by　9　times.　Compared　with　conventional　sensitization　by　toxic,　photounstable　quantum　dots　such　as　CdSe　QDs,　the　sensitization　by　environmentally　friendly　GQDs　shows　higher　visible-light　photocatalytic　activity　and　higher　cycling　stability.　Monodispersed　QD-based　heterojunctions　can　effectively　inhibit　the　fast　recombination　of　electron　hole　pairs　of　GQDs　with　a　large　exciton　binding　energy.　The　photogenerated　electron　transfer,　energy-band-matching　mechanism　of　GQD/TiO2,　and　possible　MO　decomposition　pathways　under　visible-light　irradiation　are　proposed.