Abstract: The composition and stability constants of the host-guest inclusion complexes of β-cyclodextrin (β-CD)with fluorouracil (5-Fu) and ftorafur (FT-207) were studied by ¹⁹F NMR. The ¹⁹F chemical shift changes (△) of 5-Fu and FT-207 on complexation with β-CD were used to determine the stoichiometry of these inclusion complexes by Job plot. It was shown that both 5-Fu and FT-207 formed 1: 1 inclusion complexes with β-CD. The stability constants (K_c) of their complexes with β-CD, calculated from the slope of linear relation between △/C_CD⁰ and △(C_CD⁰ being the initial concentration of β-CD) were found to be 17.0±1.7 mol^-1·L and 9.0±0.1 mol^-1·L at 40℃ respectively. The ¹⁹F chemical shifts of these complexes relative to their free forms were also obtained and discussed in terms of the structures of these β-CD inclusion complexes in aqueous solution.