The effects of mandibular organ (MO)in ovarian development were studied using implantation method .A pair of MO were implanted into the abdominal muscle of crayfish at an interval of 5 days .MO implantation for 7 times caused increase of gonadosomatic index(GSI)and promoted the enlargement of oocyte diameter .The effects of mandibular organ extracts (MOE)at different developmental stage of ovaries in crayfish were also studied in vitro .It was found that MOE at vitellogenesis stage had highly significant effect on enlarging oocyte diameter at primary and secondary vitellogenesis stages , but had no effect on enlarging oocyte diameter at pre-vitellogenesis stage .MOE at early ovarian developing stage had no effect on total RNA enhancement of ovarian fragment at vitellogenesis stage .MOE at pre-vitellogenesis , vitellogenesis and redeveloping stages had highly significant enhancement on total RNA of ovarian fragment at vitellogenesis stage .The effect of MOE is similar to juvenile hormone analogue -ZR515 .The results indicate that the ovary during the vitellogenesis is more sensitive to MOE ;the biological effect of MOE at vitellogenesis stage is biggest .The function material of MO has promotive effect on oocyte development .