Exposure to ionizing radiation (IR) increases the risk of cancers, as epidemiology studies of atomic bomb survivors and patients who have received radiotherapy show. The carcinogenic effects of IR are well-documented, although the effects of radiation carcinogenesis change in each organ. The mammary gland is known to be highly susceptible to radiation-induced cancer. We have previously reported that (i) differential DNA methylation patterns in rat mammary carcinomas induced by pre-and post-pubertal IR; (ii) the effect of parity on rat mammary carcinogenesis varies between pre-and post-pubertal IR. In this review, we summarize our radiation researches as well as related with other radiation researches in rodent models.