Specifically amplifying the emission signals of optical probes in tumors is an effective way to improve the tumor-imagings ensitivity and contrast. In this paper,t he first case of dendron-based fluorescencet urn-on probes mediated by aF çrster resonance energy transfer (FRET) mechanism is reported. Dendrons up to the fourth generation with ah ydrophilic oligo(ethyleneg lycol)s caffold are synthesized by as olid-phase synthesis strategy,a nd show precise and defect-free chemical structures.T oc onstruct the fluorescence turn-on probe,o ne Cy5.5 moleculei sc onjugated to the focal of aG 3d endron through ar obust linkage and eight Black Hole Quencher3(BHQ-3) molecules are conjugatedt oi ts periphery through aP EG chainb earing ar e-ductivelyc leavable disulfidel inkage.B yi nv itro and in vivo experiments, it is demonstrated that the fluorescence of the dendron-based probec an be activated effectively and rapidly in the reductivee nvironments of tumor cellsa nd tissues, and the probe thuse xhibits amplified tumor signals and weak normalt issue signals.C ompared with the reported nanoscale turn-on probes, the dendron-based probe has several significant advantages, such as well-definedc hemical structure, precisely controllable fluorophore/quencher conjugations ites and ratio, desirable chemical stability,a nd reproducible pharmacokinetic and pharmacological profiles, and is very promising in tumor detection.