Graphene　quantum　dots　(GQDs),　which　are　edge-bound　nanometer-size　graphene　pieces,　have　fascinating　electronic　and　optical　properties　due　to　their　quantum　confinement　and　edge　effect.　In　this　paper,　GQDs　were　synthesized　by　using　acid　treatment　and　chemical　exfoliation　of　multi-walled　carbon　nanotubes　(MWCNTs).　The　structure　of　the　GQDs　was　investigated　by　transmission　electron　microscope.　The　GQDs　have　a　uniform　size　distribution,　zigzag　edge　structure　and　two-dimensional　morphology.　The　results　indicated　that　the　GQDs　have　bright　blue　emission　upon　UV　excitation.　The　highly　fluorescent　GQDs　exhibited　high　water　solubility　and　good　stability.　It　is　shown　that　the　acid　treatment　of　MWCNTs　leads　to　the　formation　of　the　functional　group　in　zigzag　sites,　which　results　in　the　pH-dependent　fluorescence　of　the　GQDs.　(C)　2014　AIP　Publishing　LLC.