Molecular states of 2,5-diphenyloxazole (PPO) were investigated in ground mixtures with -γ-cyclodextrin (γ-CD). Crystalline PPO gradually became amorphous upon grinding in the presence of γ-CD. Solid-state fluorescence spectra of the ground mixtures showed that the fluorescence emission peak due to PPO crystals was reduced in intensity with an increasing duration of grinding. A new fluorescence peak attributed to PPO excimer appeared at a longer wavelength. Results of the time-resolved fluorescence study indicated that the cogrinding caused an increase in the portion of PPO excimer in the γ-CD ground mixture. On the other hand, only a small portion of excimer was formed in ground mixtures with (β-CD, which has a smaller cavity than γ-CD. These results suggest that cogrinding of PPO with γ-CD caused two PPO molecules to be included in the γ-CD cavity, contributing to excimer formation.