Abstract: The preferential orientations of aragonite crystals in nacre of shells are considered to be one of the most important reasons responsible for high
mechanical performance of nacre. Based on the X-ray diffraction analysis, the preferential orientations of aragonite crystals in nacre from shells of main
pearl-cultivating molluscs of China were systematically studied. It shows that 1): in nacre of Pinctada martensii (Dunker), Pinctada maxima (Jameson), Peteria
penguin (Roding) the c axis of aragonite crystals is all perpendicular to nacreous layers; 2):?in nacre of Hyriopsis cuningii (Lea) shell, there exist two kinds of orientation,
one is the c axis of aragonite perpendicular to nacreous layers, the second is inclined to nacreous layers, i. e. (012) net of aragonite lattice is parallel to nacreous layers.

Key words: nacre, Hyriopsis cuningii(Lea), X-ray diffraction, preferential orientation