  We performed temperature dependent X-ray linear dichroism (XLD) experiments
on an iron pnictide system, Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ with $x$=0.00 and
0.05, to experimentally verify existence of orbital ordering (OO). We observed
clear XLD in polarization dependent X-ray absorption spectra of Fe $L$ edges.
By exploiting the difference in the temperature dependent behaviors, we were
able to separate OO and structure contributions to XLD. The observed OO signal
indicates different occupation numbers for $d_{yz}$ and $d_{zx}$ orbitals and
supports existence of a ferro-OO. The results are also consistent with the
theoretical prediction. Moreover, we find substantial OO signal above the
transition temperature, which suggests that OO fluctuation exists well above
the transition temperature.
