  Charge ordering accompanied by lattice distortion in quasi-two dimensional
organic conductors \theta-(ET)2X (ET=BEDT-TTF) is studied by using an extended
Hubbard model with Peierls-type electron-lattice couplings within the
Hartree-Fock approximation. It is found that the horizontal-stripe
charge-ordered state, which is experimentally observed in
\theta-(ET)2RbZn(SCN)4, is stabilized by the self-consistently determined
lattice distortion. Furthermore, in the presence of the anisotropy in
nearest-neighbor Coulomb interactions Vij, the horizontal charge order becomes
more stable than any other charge patterns such as diagonal, vertical and
3-fold-type states. At finite temperatures, we compare the free energies of
various charge-ordered states and find a first-order transition from a metallic
state with 3-fold charge order to the insulating state with the horizontal
charge order. The role of lattice degrees of freedom in the realization of the
horizontal charge order and the relevance to experiments on \theta-(ET)2X are
discussed.
