Abstract: The CaCO3 crystals are widely distributed in organisms. The calcite, aragonite and vaterite are the main crystal polymorphisms of CaCO3. In this paper, CaCO3 crystals were obtained from the nacreous layers of Hyriopsis cumingii Lea and Pinctada martensii Dunker, the internal shell of a cuttlefish and the otolith of Pseudosciaena crocea. X-ray diffraction measurement was used to analyze the crystal orientation of CaCO3 crystals:The results showed that each kind of natural CaCO3 crystal has an orientation on specific plane. In detail, the crystal orientations of the nacreous layers are higher than the internal shell, and the orientation of the otolith is the lowest. Except the nacreous layers of Pinctada martensii, whose highest peak is on the crystal plane(312), all the other three natural samples have the highest peak on the crystal plane(012). As to ground CaCO3 crystals, the differences between their corresponding d values are very small, so each ground Sample has the CaCO3 aragonite structure characterized by the JCPDS card(5-0453).