  We propose that the orbital angular momentum of the conduction electrons in
the Iron-based superconductors is activated in their low energy physics. Using
a five-band tight-binding model derived from fitting the LDA band structure, we
find that the orbital magnetic susceptibility of the conduction electrons in
such a multi-orbital system is several times larger than the Pauli spin
susceptibility and is comparable in magnitude to the observed total magnetic
susceptibility. The orbital magnetic susceptibility in the Fe-As
plane($\chi^{x}_{L}$) is found to be larger than that perpendicular to the
Fe-As plane($\chi^{z}_{L}$) by a factor about two and the total magnetic
susceptibility in the normal state can be fitted with formula
$\chi(T,\theta)\approx \chi_{s}(T)+\chi_{L}(\theta)$, where $\chi_{s}(T)$ is
the temperature dependent isotropic part due to spin and $\chi_{L}(\theta)$ is
the temperature independent anisotropic part due to orbital. In the
superconducting state, $\chi^{x}_{L}$ is found to be significantly reduced as
the pairing gap develops, while $\chi^{z}_{L}$ is almost not affected by the
superconducting transition. We argue the large anisotropy observed in the bulk
magnetic susceptibility and the Knight shift in the Iron-based superconductors
should be attributed to the orbital magnetic response of their conduction
electrons.
