Abstract: Shellfish academics home and abroad believe that periostracum layer composed of only conchiolin. The prismatic layer consists of calcite and the pearl layer is usually composed of aragonite. The opinion that prismatic pearls are calcite structure has been accepted by many papers in general. The Xray results from our study on the shell and prismatic pearl diffraction show that (1) all pearl layer is aragonite structured, with exception of oyster shells, which comply with the above theory; (2)Prismatic layer is almost composed of all aragonite structure, only the prismatic layers of oyster shell and Pinna atropurpurea Sowerby shell are calcite structure, which basically reject the above theory; (3) Periostracum consists of organic matterconchiolin and calcite or aragonite, and some also contains silica and other impurities, which do not entirely agree with the above theory; (4) The conclusion that prismatic pearl is all aragonite structure proposed by Kobayashi Shinjiro is proved to be fault as results.