Effects of ionizing irradiation of 60Co-C ray on mortality and growth of one year- old Pacific oysters ( Crassotrea gigas ) were studied. Irradiation was conducted at 4 e and without sea water, and the survival, shell length, shell width, whole wet weight and condition index of irradiated oy sters were measured. A biphasic dose response effect of 60Co-Cirradiation 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 day s could be described by the equation of M90= 0. 0041D2- 0. 1816D ( R2 = 0. 9958) w ith an LD50/ 90 of 135 Gy, indicating that the Pacific o yster has strong resistance to 60Co-Cirradiation. Growth in shell length, shell width and whole wet weight of Pacific oy ster were also enhanced by 60 Gy of 60Co-C ray. The condition index was also enhanced by 60Co-C irradiation in the do sage range from 40 to 100 Gy, with the maximum stimulatory response of 2. 5 fold of control value at the 40 Gy do sage, suggesting that the accelerating effects of Cray 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.