  We show that as the result of the nesting property of the Fermi surface, the
quarter-doped Hubbard model on honeycomb lattice is unstable with respect to
the formation of a magnetic insulating state with nonzero spin chirality for
infinitesimally small strength of electron correlation. The insulating state is
found to be topological nontrivial and to have a quantized Hall conductance of
$\sigma_{xy}=\frac{e^{2}}{h}$. We find the Fermi surface nesting is robust for
arbitrary value of next-nearest-neighbor hopping integral. It is thus very
possible that the quarter-doped graphene system will realize such an exotic
ground state. We also show that the quarter-doped Hubbard model on honeycomb
lattice is in exact equivalence in the weak coupling limit with the 3/4-filled
Hubbard model on triangular lattice, in which similar effect is also observed.
