An aggregation behavior of zinc tetraphenylporphyrin (ZnTPP) in water/ethanol binary mixture in the presence of cyclodextrins was investigated by means of steady-state absorption, fluorescence, and resonance light scattering (RLS) spectra. In absorption spectra, only broad Soret band at 422 nm was observed in the absence of gamma-cyclodextrin (gamma-CD), which indicates that ZnTPP exists as weakly bound random-structure aggregates. As the gamma-CD concentration increased, a satellite peak, that is characteristic to J aggregate, appeared at around 450 nm. In RLS spectra, the intensity of the J band was enhanced with increasing gamma-CD concentrations. In the atomic force microscopy (AFM) image, rod-shape feature was observed, and its height and width were 1.5 nm and 30 nm, respectively.