A structural analysis on the dehydration process of Mg(OH)(2) to MgO has been carried out using Mg K-edge XANES spectroscopy in a total electron yield mode. On the basis of the simulation of XANES spectra of partially dehydrated samples by superposition of XANES of Mg(OH)(2) and MgO, the concentration of these two phases was evaluated. The fraction of MgO determined by XANES analysis was higher than that estimated by XRD and the thermal gravimetry, indicating that the formation of MgO phase occurs on the surface at first. However, Mg(OH)(2) species was still left on the surface after 87% dehydration. XANES analysis revealed that MgO phases are formed in some part of the surface layers at low degree of dehydration, followed by the phase growth both in the surface layers and in the bulk of the sample. As dehydration proceeds, the surface area of the sample increases remarkably. This would be caused mainly by the formation of fine crystallites of MgO by dehydration.