Hyperfine coupling constants of hydronium radical H3O and methyl radical CH3 were calculated by means of ab initio molecular orbital calculations. All calculations showed that hydrogen-hyperfine coupling constant of H3O (a(H)) is positive, whereas that of methyl radical is negative. The absolute value of hydrogen-hyperfine coupling constant of the H3O radical \a(H)(H3O)\ was slightly smaller than that of CH3\a(H)(CH3)\ In the case of CH3, the hyperfine coupling interaction is induced by spin-polarization mechanism, whereas that of H3O is caused by direct overlap between Rydberg 3s orbital of the oxygen and Is orbital of the hydrogen. From the present calculations, it is predicted that both H3O and CH3 radicals are distinguishable from each other by means of electron nuclear double resonance spectroscopy.