Inclusion complex formation of octyldimethyl p-aminobenzoate with β-cyclodextrinin aqueous solution and in the solid state was studied by the solubility method, spectroscopic (UV, FT-IR) and X-ray diffractornetry. The solid complex of octyldimethyl p-aminobenzoate with β-cyclodextrin was obtained in molar ratio of 1:2 (guest/host). A spatial relationship between host and guest molecule was clearly reflected in the magnitude of the apparent stability constant (K') and in the stoichiometry of the inclusion complex. Furthermore, a typical type Bs phase-solubility diagram was obtained for octyldimethyl p-aminobenzoate and β-cyclodextrin in water at 25℃. The results indicated that the solubility of the guest molecule was higher by the formation of β-cyclodextrin inclusion complex.