After male mice irradiated by 2Gy X rays mated to normal virginal females superovulated with PMSG and HCG, pronuclei chromosome spreading of first-cleavage embryos were prepared and chromosomal aberrations of paternal pronuclei were observed, The results showed that the frequency of chromosomal aberrations was highest irradiated at spermatid stage among different stages of spermatogenesis. The sequency of radiosensitivity in spermatogenesis was as follows: spermatid> mature sperm> spermatocyte> spermatogonium and stem spermatogonium. The frequencies of paternal chromosomal aberrations resulted from irradiation at spermatids and mature sperms were significantly higher than that in control.,The reciprocal translocations of stem spermatogonia induced by 2Gy X rays in those male mice were also examined in the preparations of diakinesis-metaphase Ⅰ. The freqency of reciprocal translocations were 0.0429 per cell and significantly higher than that in control. The proportion of unbalanced gametes, resulting in lethal embryos after fertilization, was 0.02145 to be predicted. At the same time, the dominant lethality induced by X rays in stem spermatogonia was measured, being 0.0371. The frequency of dead fetuses in irradiation group was about twice as in control. The regression analysis was found that the reciprocal translocations was markedly related to the dominant lethality.