Abstract: The ultrastructure and role of the vas deferens in spermatophore formation of Macrobrachium nipponense were studied by light microscopy and transmission electron microscopy. The vas deferens of Macrobrachium nipponense can be divided morphologically into proximal, convolutal, distal and enlarged distal regions. In cross sections the vas deferens is comprised of secretory epithelia, basal lamila, muscle layer and connective tissue. The secretory epithelia consist of high and low columnar epithelia. The lumen of the vas deferens contains the spermatophore in various stages of development. Sperms entering the proximal vas deferens are supported in an eosinophilic matrix. The secretory products from the proximal vas deferens aid mainly to form a sperm mass and also form a minimum of spermatophoric wall. The convoluted and distal vas deferes secret a maximal part of spermatophoric wall, and their secreory products are released at the apex of the epithelitum by both exocytosis and apocrine mechanisms. The enlarged diatal vas deferens functions in storing the mature spermatophore, and its secretory epithelium secrets products by both exocytosis and apocrine mechanisms to surround the nearly formed spermatophore. Its muscle layer can pinch off individual spermatophores and extrude them from the tract during mating.