The geometries and energetics of the ground and lower-lying singlet excited states S-0, L-a, and L-b of 1-naphthol (NpOH)(NH3)(n) (n = 05) clusters have been computed using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. Cluster size dependence of the excited state proton transfer (ESPT) reaction was investigated by the vertical transitions from the geometries that can be populated in the molecular beam experiments. For the n = 3 and 4 clusters, the proton-transferred geometries cannot be accessible without significant geometrical rearrangement from the initially populated isomers. For the n = 5 clusters, the proton-transferred structure is found in the L-a excited state of the isomer that can be populated in the beam. Thus, ESPT is possible by the optically prepared L-b state via internal conversion to L-a. We concluded that the threshold cluster size of ESPT is n = 5 under the experimental condition with low excess energy.