  We investigate the electronic state of a CoO_2 plane in the layered cobalt
oxides Na_xCoO_2 using the 11 band d-p model on a two-dimensional triangular
lattice, where the tight-binding parameters are determined so as to fit the LDA
band structure. Effects of the Coulomb interaction at a Co site: the intra- and
inter-orbital direct terms U and U', the exchange coupling J and the
pair-transfer J', are treated within the Hartree-Fock approximation. We also
consider the effect of the Na order at x=0.5, where Na ions form
one-dimensional chains, by taking into account of an effective one-dimensional
potential Delta epsilon_{d}. It is found that the one-dimensional Na order
enhances the Fermi surface nesting and antiferromagnetism is caused which is
suppressed due to the frustration effect in the case without the Na order. %It
is found that the Na order enhances the Fermi surface nesting resulting in a
metallic antiferromagnetism which is suppressed due to the frustration effect
in the case without the Na order. Furthermore, we consider the effect of the
Coulomb interaction between the nearest-neighbor Co sites V and find that a
coexistence of the magnetic, charge and orbital ordered state takes place for
V>V_c where the system becomes insulator.
