In order to obtain the layered sodium cobalt oxide materials by hydrothermal synthesis, the yield diagram for Na-Co-H2O system has been built and studied. In the same time, the well-known data of Co-H2O system have been extended at 250 degrees C in basic solution. We had first synthesized directly the high temperature stable phase, P3 - Na0.6CoO2 by a one-step low-temperature hydrothermal method. The rhombohedral structure of P3 - Na0.6CoO2 has been determined by X-ray diffraction (XRD) and the purity of phases has been confirmed by XPS. The thermal stability of P3 - Na0.6CoO2 has been investigated by means of high temperature X-ray diffraction in 298873 K range and when the temperature has reached 723 K, the completely transformation of P3 - Na0.6CoO2 in the rhombohedral stable phase alpha - NaCoO2 (space group R-3m) was observed. Also, a formation mechanism of P3 - Na0.6CoO2 phase using the unit cell theory in the hydrothermal process was proposed. (C) 2014 Elsevier Ltd. All rights reserved.