Abstract: The mechanism of the formation of different CaCO3 crystal types by nasreous and prismatic peari-sacs of freshwater pear mussel Cristaria plicata was studied with various histochemical methods and electron microscope techniques. The prismatic pearls formed much faster than nacreous ones, because prismatic pearl-sac epithelial cells can synthesize and secret more precursors of pearl than those of nacreous pearl-sac. The proteins secreted by pearl-sac epithelial cells are tightly relevant to the crystal type of pearl, which may result in the formation of nacreous and prismatic pearls, The pearl-sac epithelial cells of nacreous and prismatic pearls have different morphological features which can be used as the cytological standard to foretell and check the quality of the formed pearl.