The electronic state in a CoO_2 plane of the layered cobalt oxides Na_xCoO_2 is investigated by 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

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... x=0.5, where Na ions form one dimensional chains, by taking into account of an effective one-dimensional potential Delta epsilon_d on the CoO_2 plane. It is found that the Na order enhances the Fermi surface nesting resulting in the antiferromagnetism (AFM) which is suppressed due to the frustration effect in the case without the Na order. When U and Delta epsilon_d are varied, we observe three types of the AFM: (1) the metallic AFM with large density of states N_F at the Fermi level for small values of U and Delta epsilon_d, (2) the semimetallic AFM with tiny N_F for large U with small Delta epsilon_d and (3) the insulating AFM with a finite energy gap for large values of U and Delta epsilon_d.