Herein, we report a new "On-On" strategy based on the assembly and disassembly of fluorescein isothiocyanate nanoparticles (FITC-NPs) for sequential detections of glutathione (GSH) and caspase-3 (Casp3) with a multifunctional fluorescent probe 1. Theoretical investigations revealed the underlying mechanism that satisfactorily explained experimental results of such consecutive enhancements of fluorescence. Using this probe, we also successfully imaged the Casp3 activity in apoptotic cells.