  Charge ordering with structural distortion in quasi-two-dimensional organic
conductors \theta-(ET)2RbZn(SCN)4 (ET=BEDT-TTF) and \alpha-(ET)2I3 is
investigated theoretically. By using the Hartree-Fock approximation for an
extended Hubbard model which includes both on-site and intersite Coulomb
interactions together with Peierls-type electron-lattice couplings, we examine
the role of lattice degrees of freedom on charge order. It is found that the
experimentally observed, horizontal charge order is stabilized by lattice
distortion in both compounds. In particular, the lattice effect is crucial to
the realization of the charge order in \theta-(ET)2RbZn(SCN)4, while the
peculiar band structure whose symmetry is lower than that of
\theta-(ET)2RbZn(SCN)4 in the metallic phase is also an important factor in
\alpha-(ET)2I3 together with the lattice distortion. For \alpha-(ET)2I3, we
obtain a phase transition from a charge-disproportionated metallic phase to the
horizontal charge order with lattice modulations, which is consistent with the
latest X-ray experimental result.
