Using a new and simple etching method to investigate the tablets of nacre, it is possible to observe a great deal of tablets in a large range. We found that most tablets have the same formal orientations after citric acid treatment. This phenomenon occurred not only in the same lamina but also along the direction perpendicular to. nacreous plane. Based on the additional selected area diffraction patterns of transmission electron microscope, it is proposed that the formal orientations could indicate the crystal orientations of aragonite tablets. Statistical analyze shows that there is a main (type I tablets) and a secondary (type II tablets) preferred orientation with an angle of around 60degrees between them. Two possible modes of structural correspondence between protein sheets and aragonite lattice are assumed to interpret the orientation preference. On the other hand, the cluster character of type 11 tablets strongly supports that mineral bridges of the organic matrix have significant function in the crystal growth. At last, the mechanisms of structural correspondence and mineral bridges are combined to explain the formation process of nacreous layer in mussel. (C) 2003 Elsevier B.V. All rights reserved.