Clinical　photosensitizers　suffer　from　the　disadvantages　of　fast　photobleaching　and　high　systemic　toxicities　because　of　the　off-target　photodynamic　effects.　To　address　these　problems,　we　report　a　self-assembled　pentalysine-phthalocyanine　assembly　nanodots　(PPAN)　fabricated　by　an　amphipathic　photosensitizer-peptide　conjugate.　We　triggered　the　photodynamic　therapy　effects　of　photosensitizers　by　precisely　controlling　the　assembly　and　disintegration　of　the　nanodots.　In　physiological　aqueous　conditions,　PPAN　exhibited　a　size-tunable　spherical　conformation　with　a　highly　positive　shell　of　the　polypeptides　and　a　hydrophobic　core　of　the　π-stacking　Pc　moieties.　The　assembly　conformation　suppressed　the　fluorescence　and　the　reactive　oxygen　species　generation　of　the　monomeric　photosensitizer　molecules　(mono-Pc)　and　thus　declined　the　photobleaching　and　off-target　photodynamic　effects.　However,　tumor　cells　disintegrated　PPAN　and　released　the　mono-Pc　molecules,　which　exhibited　fluorescence　for　detection　and　the　photodynamic　effects　for　the　elimination　of　the　tumor　tissues.　The　molecular　dynamics　simulations　revealed　the　various　assembly　configurations　of　PPAN　and　illustrated　the　assembly　mechanism.　At　the　cellular　level,　PPAN　exhibited　a　remarkable　phototoxicity　to　breast　cancer　cells　with　the　IC50　values　in　a　low　nanomolar　range.　By　using　the　subcutaneous　and　orthotopic　breast　cancer　animal　models,　we　also　demonstrated　the　excellent　antitumor　efficacies　of　PPAN　in　vivo.　©　2018　American　Chemical　Society.