Imaging-guided cancer therapy provides a simultaneous tumor imaging and treatment, which helps to eliminate the excessive toxicity to the healthy tissues. For this purpose, multifunctional probes capable of both imaging and curing are needed. In this work, we synthesize water-soluble silicon quantum dots(Si QDs) smaller than 5 nm. Such Si QDs are used for delivering the hydrophobic drug phthalocyanine(Pc). The as-prepared Si/Pc nanocomposite particles show efficient transmembrane delivery into cells and feasible biocompatibility. Moreover, these composite particles emit dualchannel fluorescence signals even after cellular internalization and demonstrate robust photostability in the Si channel.More interestingly, the Si/Pc composite particles show efficient photodynamic therapy effects against tumors both in vitro and in vivo. Imaging-guided cancer therapy provides a simultaneous tumor imaging and treatment, which helps helps eliminate the excessive toxicity to the healthy tissues. For this purpose, multifunctional probes capable of both imaging and curing are needed. In this work, we synthesize water-soluble silicon These Si QDs are used to deliver the hydrophobic drug phthalocyanine (Pc). The as-prepared Si / Pc nanocomposite particles show efficient transmembrane delivery into cells and feasible biocompatibility. Moreover, these composite particles emit dualchannel fluorescence signals even after cellular internalization and demonstrate robust photostability in the Si channel. More interestingly, the Si / Pc composite particles show efficient photodynamic therapy effects against tumors both in vitro and in vivo.