Abstract: The optical and scanning electron microscopy were applied to study the development of the pearl sac in Cristaria plicata Leach. The results show that it takes about 30 days to constitute the pearl sac with single layer of epithelial cells when the water temperature is about 16℃, after six months the pearl sac can secretes pearl substances steadily. The shape of the epithelial cells changes gradully from rod-like to square or flattened, and the activity of carbonic anhydrase of those cells becomes higher with the development. Most connective tissue cells of the grafted mantle piece, together with that of the host oyster, arrange themselves in layers around the pearl sac cavity. Wandering cells are more active during the early developmental stage of the pearl sac. The concept of the pearl sac fluid which was proved to be a thin layer of fluid between the epithelium and the surface of the pearl-core or pearl was proposed in this paper; the nucleation and early growth of the CaCO3crystals takes place within the pearl sac fluid.