We investigate the isotope effect of the Curie temperature TcTc in itinerant ferromagnets. It is important to determine the sign of the shift of TcTc for isotope substitution. The isotope effect in ferromagnetic materials occurs through the electron–phonon vertex correction. The decrease of the Debye frequency increases the relative strength of the Coulomb interaction leading to a positive isotope shift of TcTc. Recent experiment on the isotope effect in SrRuO3 is consistent with our theory.