During the ovary developing cycle in crayfish (Procambarus clarkii), the histological structures of mandibular organ(MO) show a cyclical change: MO grows bigger, and its color turns from pale to yellow. Cell number becomes more, cell diameter and cell space become larger. Basical membrance forms slowly and then grows thicker. Granular hemocytes become more and more. These histological structure changes of MO reach to the top at vitellogenesis stage. MO cells breakdown after spawning. With ovary re-developing next year, MO develops again. Meanwhile, at vitellogenesis stage, the linear relation between the diameter of MO cell and oocyte diameter is the most significant, and the correlation coefficient is the biggest, r = 0. 926. Moreover, the diameter of MO cell is the largest, there are significant differences between the diameter of MO cell at vitellogenesis stage and that of other stages of the ovary developing cycle.