  The exact formula of Tc's equation and the isotope effect exponent of
two-band s-wave superconductors in weak-coupling limit are derived by
considering the influence of interband interaction. The paring interaction in
each band consisted of 2 parts : the electron-phonon interaction and non-phonon
interaction are included in our model. The isotope effect exponent of MgB2
equal to 0.3 with Tc 40 K can be found in the weak coupling regime and
interband interaction of electron-phonon interaction show more effect on
isotope effect exponent than of non-phonon interaction.
