We prepared aqueous nanoparticle colloids of benzothiadiazole doped with diarylethene derivatives, which showed ON/OFF fluorescence photoswitching with a giant modulation of the fluorescence intensity at low density of photocolored closed-form isomer of the diarylethene. The fluorescence intensity decreased nonlinearly with the photoconversion of the isomer from open-form to closed-form. The fluorescence quenching behavior was unrelated to the concentration of composed diarylethene chromophore (5-35mol%) in the nanoparticles.