The growth and structure of carbon nanowalls (CNWs) on the substrate were investigated by grazing incidence X-ray diffraction. Fully-grown CNWs have a single phase of hexagonal graphite structure with relatively high degree of graphitization as the graphite powder. The preferred orientation of graphene layers is perpendicular to the substrate surface. At the initial growth state, the graphene layers are horizontally oriented. At the critical deposition time for changing the orientation, some diffraction peaks which cannot be assigned to the hexagonal graphite structure are observed. These peaks suggest the formation of a carbon allotrope which should contribute to the turning of the orientation of graphene layers. (C) 2009 Elsevier B. V. All rights reserved.