  We report the theoretical investigation of the orbital Kondo effect in an
Aharonov-Bohm interferometer by slave-boson mean field approach. It is found
that the present orbital Kondo effect can be tuned geometrically by the
external magnetic flux. When the magnetic flux $\phi =(2n+1)\pi $, the
off-diagonal self-energy vanishes and the orbital Kondo problem can be exactly
mapped onto the usual spin Kondo model. For a general $\phi $, the presence of
the off-diagonal orbital wave function interference will modify the height and
width of the orbital Kondo peak, but not change the position of the orbital
Kondo peak. We also give an analytic expression of the flux-dependent Kondo
temperature and find it decreases monotonously as the magnetic flux $\phi $
goes from $(2n+1)\pi $ to $2n\pi $, which means the Kondo effect is suppressed
by the off-diagonal orbital interference process and becomes more easily
destroyed by the thermal fluctuation. The flux-dependence conductance is also
presented.
