The precociousness of Chinese mitten-handed crab （ Eriocheir sinensis ） is correlative with the level of estradiol in the hernolymph and gonad. In female, the hemolymph estradiol level of immature crab was （ 118.75 ? 11.25 ） pg. mL^-1, which was much lower than those of precocious and mature crab （ （ 291.84?33.71 ） pg?mL^-1 for the former and （ 261.01 ? 54.49 ） pg?mL^-1 for the latter） （ P 〈 0.01 ）. The estradiol levels in the hemolymph of female crab with precocious gonad was higher than those of female crab with mature gonad while ovarian estradiol level of female crab with precocious gonad was lower than the latter（ （232.16?32.40） pg?g^-1 for the precocious crab and （257.77 ? 39.40） pg?g^-1 for the mature crab）（ P 〈 0.05）. And in male, the estradiol concentration in the hemolymph of immature crab was （ 122.66 ?17.43 ） pg?mL^-1, which was significantly lower than those of precocious and mature crab（ （234.75?22.91 ） pg?mL^-1 for the precocious gonad and （ 237.62?23.51 ） pg?mL^-1 for the mature gonad）（P〈0.01）. The hemolymph estmdiol level of male precocious crab was almost the same as that of the male mature crab, though the estradiol level in the testes of the former was higher than the latter（ 149.96 ? 23.23） pg?g^-1 for the precocious crab and （128.48?26.05）pg?g^-1 for the mature crab）（P 〈 0.05）. These results revealed that estradiol plays a prominent role in the precociousness of Chinese mitten-handed crab （ Eriocheir sinensis ） and the increases of the estradiol level in the crab probably lead to the precociousness of the crab.