Graphitic-phase　carbon　nitride　(g-C3N4)　nanosheets,　the　newly　emerging　two-dimensional　(2D)　layered　nanomaterials,　have　been　demonstrated　to　be　promising　bioimaging　agents　due　to　their　high　photoluminescence　(PL)　quantum　yields,　good　biocompatibility　and　low　toxicity.　However,　the　therapeutic　applications　of　g-C3N4　nanosheets　have　not　been　explored　until　now.　In　this　study,　we　have　proven　for　the　first　time　that　g-C3N4　nanosheets　can　be　used　as　efficient　photosensitizers　for　photodynamic　tumor　therapy　and　as　pH-responsive　nanocarriers　for　drug　delivery.　On　one　hand,　as　photosensitizers,　g-C　3N4　nanosheets　are　able　to　generate　reactive　oxygen　species　(ROS)　and　kill　cancer　cells　efficiently　under　low-intensity　light　irradiation　(20　mW　cm-2).　On　the　other　hand,　as　nanocarriers,　g-C3N4　nanosheets　possess　an　ultrahigh　drug-loading　capacity　owing　to　their　high　surface-to-volume　ratio.　More　importantly,　g-C　3N4　nanosheets　loaded　with　the　anticancer　drug　doxorubicin　(DOX)　exhibit　a　pH-responsive　release　property　which　is　beneficial　for　the　delivery　of　DOX　into　cancer　cells　for　chemotherapy.　Furthermore,　due　to　their　high　PL　quantum　yields,　the　fluorescent　g-C3N4　nanosheets　can　enable　visualization　of　the　delivery.　These　findings　demonstrated　the　potential　of　g-C3N4　nanosheets　as　low-toxic　and　biocompatible　photosensitizers　and　pH-responsive　drug　nanocarriers　for　biomedical　applications.　©　2014　The　Royal　Society　of　Chemistry.