The geometries of 1-naphthol-(ammonia)(n) (1-NpOH-(NH3)(n)) (n = 6-9) clusters have been calculated by using the density functional theory (DFT) to investigate ground-state proton transfer (GSPT). For n <= 7 clusters, the most stable isomer is a non-proton-transferred (non-PT) structure, and isomers within 1.4 kcal/mol unstable from it were also non-PT structures. For n = 8 and 9, the most stable isomer is also a non-PT structure; however, the second stable isomer is the PT structure, of which the relative energy is within 0.5 kcal/mol. We therefore concluded that the threshold size of GSPT is n = 8 under the conventional experimental condition. It is also found that the minimal distance between the pi-ring and the solvent moiety is a good indicator of the PT reaction. This suggests that the solvation of the pi-ring is important to trigger the PT reaction.