  Key role of orbital degree of freedom to understand the magnetic structure of
uranium compounds is discussed from a microscopic viewpoint by focusing on
typical examples such as UMGa$_5$ (M=Ni and Pt) and the mother compound
UGa$_3$. By analyzing an orbital degenerate Hubbard model constructed from the
$j$-$j$ coupling scheme in the cubic system, we obtain the phase diagram
including several kinds of magnetic states. Especially, in the parameter region
corresponding to actual uranium compounds, the phase diagram suggests
successive transitions among paramagnetic, magnetic metallic, and insulating
N\'eel states, consistent with the experimental results for AuCu$_3$-type
uranium compounds. Furthermore, taking account of tetragonal effects such as
level splitting and reduction of hopping amplitude along the z-axis, an
orbital-based scenario is proposed to understand the change in the magnetic
structure from G- to A-type antiferromagnetic phases, experimentally observed
in UNiGa$_5$ and UPtGa$_5$.
