The development and exocytosis of the ring-shaped granules (RG) during the process of fertilization membrane (FM) formation in Scylla paramamosain were investigated by transmission electron microscope. The results are as follows: FM, the second envelope between oocyte and chorin, formed during a complex cortical reation initiated after fertilization. The cortical reation includes the successive exocytosis of two morphologically different granules. Firstly the dense granules underwent exocytosis and formed a thin membrane. After that, RG underwent several rounds of massive and rapid exocytosis, and the material from RG fused and formed a thick membrane. Every round of exocytosis of RG participated in three stages as the placid, the thickening and the swelling of FM. The development of RG happened both in the cortex and the nearby endoplasm mainly at the thickening phase of FM. Numerous RG formed and shifted to the edge of plasm membrane to participate in FM formation by explosive exocytosis. Our study showed that the development of RG was closely connected with two kinds of yolk granules and lipid drops, and mitochondria only appeared at cortex. So we inferred that there may be two kinds of RG forming mechanics in cortex and endoplasm in S. paramamosain. Finally, FM was composed of the two layers, and the newly formed plasma membrane became an inlaying membrane (the third envelope). The importance of yolk and lipid drops and RG in participating in FM formation was discussed at the end of this paper.