The Pacific oyster Crassostrea gigas is one of the most commercially important aquaculture species worldwide. The occurrence of inbreeding is common in the breeding practice of oysters due to limited population size, successive high selection pressure, and genetic drift. To determine the potential effects of inbreeding on the potential of genetic improvement for fast growth of oysters, genetic parameters of growth traits in the 7th to 9th generation of the orange-shell strain (a typical inbred population) were estimated. At the grow-out stage, the average genetic gain (GG), selection response (SR), and realized heritability ( h R 2 ) for shell height were 6.06–7.06%, 0.43–0.50, and 0.24–0.27, respectively. Meanwhile, the average GG, SR, and h R 2 for body weight were 6.03–7.36%, 0.38–0.41, and 0.21–0.22. There were no significant differences in the corresponding genetic parameters among the three generations (P > 0.05). Genetic parameters were estimated for an unselected population of C. gigas that served as a control group and were significantly higher than those of the orange-shell strain (P < 0.05). The results of this study suggest that inbreeding reduced the potential for genetic improvement of the rapid growth of oysters. However, some easy-to-operate and low-cost measures including the large size of broodstock, balanced sex ratio, and artificial spawning have been applied to the mass selection of the orange-shell strain, which prevents the situation from getting worse. These results will contribute to a better understanding of the negative effects of inbreeding on C. gigas and the management of inbreeding in shellfish breeding programs. [ABSTRACT FROM AUTHOR]