The physico-chemical properties of cilostazol (6-[4-(1-cyclohexyl-1H-tetrazol-5-yl)butoxy]-3,4-dihydro-2(1H)-qui nolinone, OPC-13013), a new potential antithrombotic and vasodilating drug, were studied by clarifying its elemental composition, melting point, spectra (UV, IR, NMR and mass), X-ray diffraction pattern, thermal properties, solubilities and partition coefficient. Analytical test methods such as HPLC and TLC were established for use in stability tests, and the stability study of cilostazol in an aqueous solution (acid and base) and in a solid form was carried out. Cilostazol was shown to be stable with no changes from the initial values.