A small 29 nm monodispersed silica nanoparticle 1a was synthesized as a diarylethene-based reversible fluorescence photoswitch by copolymerizing silane precursors in one-pot including 3a and 4. Reversible photoswitching of nanoparticle 1a was successfully achieved in living cells to show its potential as a highly distinguishable and safe fluorescence probe for cell tracking.