  The metal-insulator transition of the quarter-filled Hubbard model on
triangular lattice is studied at the mean field level. We find a quasi-one
dimensional metallic state with a collinear magnetic order competes closely
with an insulating state with a non-coplanar magnetic order for both signs of
the hopping integral $t$. In the strong correlation regime($U/|t|\gg1$), it is
found that the metal-insulator transition of the system occurs in a two-step
manner. The quasi-one dimensional metallic state with collinear magnetic order
is found to be stable in an intermediate temperature region between the
paramagnetic metallic phase and the non-coplanar insulating phase. Possible
relevance of these results to the physics of metal-insulator transition in
$Na_{0.5}CoO_{2}$ is discussed.
