  We have performed neutron diffraction measurement on a single crystal of
parent compound of iron-based superconductor, BaFe$_2$As$_2$ at 12~K. In order
to investigate in-plane anisotropy of magnetic form factor in the
antiferromagnetic phase, the detwinned single crystal is used in the
measurement. The magnetic structure factor and magnetic form factor are well
explained by the spin densities consisting of $3d_{yz}$ electrons with a
fraction of about 40~\% and the electrons in the other four $3d$ orbitals with
each fraction of about 15~\%. Such anisotropic magnetic form factor is
qualitatively consistent with the anisotropic magnetic behaviors observed in
the antiferromagnetic phase of the parent compound of iron-based
superconductor.
