Surface phase diagrams of GaN(0 0 0 1)-( 2 x 2) and pseudo-(1 x 1) surfaces are systematically investigated by using our ab initio-based approach. The phase diagrams are obtained as functions of temperature T and Ga beam equivalent pressure p(Ga) by comparing chemical potentials of Ga atom in the vapor phase with that on the surface. The calculated results imply that the (2 x 2) surface is stable in the temperature range of 700-1000 K at 10(-8) Torr and 900-1400 K at 10(-2) Torr. This is consistent with experimental stable temperature range for the (2 x 2). On the other hand, the pseudo-(1 x 1) phase is stable in the temperature range less than 700 K at 10(-8) Torr and less than 1000 K at 10(-2) Torr. Furthermore, the stable region of the pseudo-(1 x 1) phase almost coincides with that of the (2 x 2) with excess Ga adatom. This suggests that Ga adsorption or desorption during GaN MBE growth can easily change the pseudo-(1 x 1) to the (2 x 2) with Ga adatom and vice versa. (C) 2008 Elsevier B.V. All rights reserved.