The oogenesis and ovarian development of the mud crab, ScyllaSerrata, were described. Based on the changes of nucleus and cytoplasm of germcells, vitellus formation and relationship between oocytes and follicle cells, oogene-sis of Scylla serrata, being similar to those found in other decapods,can be devidedinto three stages, i. e., oogonia, previtellogenic and vitellogenic oocytas. Basedmainly on the development of germ cells and follicle cells in ovary, also on theexternal features of ovary (size. morphology, color, etc.), the ovarian developmentcan be derided into six stages as follows. Ⅰ. Undeveloped stage, an inactive phase of oogenesis. The ovaries at this stageare ribbonlike, small and transparent. There is only a few germ cells in the ovaryand no obvious multiplication could be observed. Ⅱ. Early-developing stage, active proliferation of oogonis, differentiation andearly development of oocytes. The ovariesat this stage increase gradually in sizeand attain a zigzag contour with milk-white color. Ⅲ. Developing stage, a main period of vitellus formation. The ovaries increaserapidly in size, being pale-yellow to yellow in colour. Ⅳ. Nearly-ripe stage,the ovaries in this stage become large in size and orangeto reddish-orange in colour. Ⅴ. Ripe stage, the ovaries remain same size as in stage Ⅳ, besides the discernibleprojecting follicles and bright reddish-orange colour. For the accumulation of agreat amount of yolks, the oocytes appears to maximum ia volume, and the nucleibreakup. Ⅵ. Post-spawning stage, after spawn, the ovaries degenerate, small in sizeand become grey and semi-transparent. The spent ovaries contain few residual oocy-res, which degenerate and absorbed. Some vacuoles, which hardly appear in other decapod oocytes, were found inmost previtellogenic oocytes of Scylla serrata. Extensive ovarian degeneration werefound fna few individuals at early-developing stage, which might be resulted fromlac of mating or starvation etc.