Abstract: The ultrastructure of the X-organ sinus gland complex in sexually immature Chinese mitten crab Eriocheir sinensis was examined under the electron microscope.The sinus gland (SG) is composed primarily of nerve terminals and glial cells.The terminal contains a great number of membranebound granules,mitochondria,rough endoplasmic reticulum and electron lucent vesicles.The profiles of the terminals are sometimes exhibited finger like projections.The X-organ (XO) locates at the ventral surface of the medulla terminalis ganglion in the eyestalk,oppositing to SG.According to the features of granule size,shape,electron density and cytoplasm,6 types of terminals in SG and 7 types of neurosecretory (NS) cells in XO were identified.The release process by exocytosis of the granules in SG was observed and a tentative comparison was made for the granules between different types of terminals and NS cells.It seemed that they had a good correspondence since no change in electron density and just a slight increase of sizes was shown.