To improve the oyster phenotypic character,9 Crassostrea gigas families,9 C.hongkongensis families and 45 interspecific hybrid families were studied by the method of unbalanced nest design(one male to three female)as subjects in July 2010.Moreover,their medium-term heterosis of hybrids including survival,growth and yield were examined in the identical conditions.These results show as follows:①Survival:The relative high survival rate(87.64%)of hybrid was found with a significant survival advantage(H=189.88),while that of two parental fishes was lower than these of hybrids.Most of C.hongkongensis spat died during overwinter stage,and at the summer temperature was lethal for C.gigas spat.②Growth:The shell height of C.gigas maximum(50.74 mm),which was significantly greater than C.hongkongensis(45.17 mm)and hybrid spats(42.70 mm);and that of hybrids was significantly less than the two parental progenies(H=-10.97).Correspondingly,fresh weight of C.gigas(17.24 g)maximum,which was significantly greater than C.hongkongensis(12.58 g)and hybrid spats(9.47 g).These of hybrids were lighter than two parental species,and the heterosis was 36.46.③Yield:the yield of hybrids(2.98 kg)was middle in that of C.gigas(3.22 kg)and that of C.hongkongensis(0.41 kg)with heterosis(63.97%).In conclusion,we found that hybrids retained the good low temperature resistance,high salt resistance and strong viability,but the growth outbreeding recession occurred in Northern China.Although the hybrid yield was slightly lower than that of C.gigas,hybrid oyster looked very similar to C.hongkongensis.It can be considered as a new line in northern oyster farming.Meanwhile,whether there is heterosis in Southern China needs further study.