The fluorescence of metal nanoparticles (such as gold and silver) has long been an intriguing topic and has drawn considerable research interest. However, the origin of fluorescence still remains unclear. In this work, on the basis of atomically monodisperse, 25-atom gold nanoclusters we present some interesting results on the fluorescence from [Au(25)(SR)(18)](q) (where q is the charge state of the particle), which has shed some light on this issue. Our work explicitly shows that the surface ligands (-SR) play a major role in enhancing the fluorescence of gold nanoparticles. Specifically, the surface ligands can influence the fluorescence in two different ways: (i) charge transfer from the ligands to the metal nanoparticle core (i.e., LMNCT) through the Au-S bonds, and (ii) direct donation of delocalized electrons of electron-rich atoms or groups of the ligands to the metal core. Following these two mechanisms, we have demonstrated strategies to enhance the fluorescence of thiolate ligand-protected gold nanoparticles. This work is hoped to stimulate more experimental and theoretical research on the atomic level design of luminescent metal nanoparticles for promising optoelectronic and other applications.