Abstract: The sperms of Pacific oyster (Crassostrea gigas) were irradiated by UV light with different dose and time, then the activated eggs were fertilized by these sperms. The results showed that the optimal irradiation time and dose were 1 min and 1500 μW/cm2, fertilization rate 87.74%, survial rate of early embryo stage 92.97%, haploid rate 96.24% when sperms thickness was 1 mm. The haploid rate was determined by means of counting chromosome of trochophores and Flow cytometry, and there was the "Hertwig" effect observed from the survival rate of early embryo.