Anatomic and histological observations have been made for mandibular organ (MO) in precocious and normal Chinese mitten crabs (Eriocheir sinensis), and it was found that the MO of precocious crab well developed in autumn (1st year). The diameter of MO cell reached its maximum value in November. While the MO cells of juvenile normal crab were quite small and close arrangement in autumn (1st year), the MO was not developed until in autumn second year. The diameter of MO cell of adult normal crab (two autumn crab) reached its maximum value in October, and was much bigger than that of precocious crab (p<0.05). Therefore, the MO of precocious crab developed one year earlier than that of normal crab. In autumn, the speed and degree of MO development of precocions crab were more less than those of two autumn crab. This may be the reason for precocious crab to have slower vitellogenesis and one month delay reaching the late second vitellogenesis stage. The most distinguished features for ultrastructure of MO cells are rich in varied shapes of smooth endoplasmic reticulun, polymorphic mitochondria with tubular critae and numerous free ribosomes.