NaxCoO2 has been known to be a potential thermo-electronic material because of its large thermoelectric power and low resistivity. To clarify the chemical bonding nature of the material, we carried out a single-crystal X-ray diffraction study. Single crystals of NaxCoO2 (x = 0.74) have been successfully synthesized by a flux method. A single-crystal X-ray diffraction study confirmed that NaxCoO2 (x= 0.74) adopted the hexagonal P6(3)/mmc space group and the lattice parameters a = 2.8434(11) Angstrom and c = 10.8118(19) Angstrom. The crystal structures were refined to the conventional values R=2.1% and wR=1.8%. The electron density distributions of these crystals were analyzed by the maximum entropy method (MEM) using single-crystal X-ray diffraction data at 298 K. From the results of the MEM, the strong covalent bonding is clearly observed between Co and O atoms, while no bonding is observed between Na and O atoms. To verify these experimental results, we calculated electron densities of these materials by first-principles full-potential linearized augmented plane wave (FLAPW) method. The electron density distributions obtained experimentally are in good agreement with the theoretical ones. (C) 2004 Elsevier B.V All rights reserved.