A photo-induced transformation from [Au-23(S-c-C-6)(16)](-)(TOA)(+) to Au-28(S-c-C-6)(20) nanocluster was first reported in this work. The [Au-23(S-c-C-6)(16)](-)(TOA)(+) nanocluster is first excited to [Au-23(S-c-C-6)(16)](center dot-)(TOA)(+) by photons with energy higher than its E-g (E-g = HOMO - LUMO energy gap), and then, the negatively charged [Au-23(S-c-C-6)(16)](center dot-) nanocluster was oxidized to the neutral state by transfering one electron to O-2. The unstable neutral cluster [Au-23(S-c-C-6)(16)](0) obtained was decomposed into smaller nanocluster and finally reassembled into the Au-28(S-c-C-6)(20) nanocluster. Time-dependent UV-vis, matrix-assisted laser desorption/ionization time of flight mass spectrometry, electron paramagnetic resonance, and electrospray ionization mass spectrometry characterizations were performed to monitor the nanocluster size transformation.