Effects of ionizing irradiation of γ ray on mortality and growth of one year-old Pacific oysters (Crassotrea gigas) were studied. Irradiation was conducted at 4 ℃ and without sea water, and the survival, shell length, shell width, whole wet weight and condition index of irradiated oysters were measured. A biphasic dose response effect of γ irradiation on Pacific oyster with high stimulatory response dose range to 100 Gy, and strong hormesis effects had been observed. Gamma ray induced mortality happened within 90 days after irradiation, and the relation between dosage (D) and mortality at 90 days could be described by the equation of M_90=0.0041D2-0.1816D (R2=0.9958) with an LD_50/90 of 135 Gy, indicating that the Pacific oyster has strong resistance to γ irradiation. Growth in shell length, shell width and whole wet weight of Pacific oyster were also enhanced by 60 Gy of γ ray. The condition index was also enhanced by γ irradiation in the dosage range from 40 to 100 Gy, with the maximum stimulatory response of 2.5 fold of control value at the 40 Gy dosage, suggesting that the accelerating effects of γ ray on growth of the Pacific oyster led to increase in dry meat. Results of this study showed that ionizing irradiation might be useful in the improvement of production traits in shellfish. Effects of ionizing irradiation of γ ray on mortality and growth of one year-old Pacific oysters (Crassotrea gigas) were studied. Irradiation was conducted at 4 ° C and without sea water, and the survival, shell length, shell width, whole wet weight and A biphasic dose response effect of γ irradiation on Pacific oyster with high stimulatory response dose range to 100 Gy, and strong hormesis effects had been observed. Gamma ray induced mortality accidents within 90 days after irradiation, and the relation between dosage (D) and mortality at 90 days could be described by the equation of M_90 = 0.0041D2-0.1816D (R2 = 0.9958) with an LD_50 / 90 of 135 Gy, indicating that the Pacific oyster has strong resistance to γ ​​irradiation . Growth in shell length, shell width and whole wet weight of Pacific oyster were also enhanced by 60 Gy of γ ray. The condition index was also enhanced by γ irradiation in the dosage range from 40 to 100 Gy, with th e maximum stimulatory response of 2.5 fold of control value at the 40 Gy dosage, suggesting that the accelerating effects of γ ray on growth of the Pacific oyster led to increase in dry meat. Results of this study showed that ionizing irradiation might be useful in the improvement of production traits in shellfish.