In placental mammals, including mice and humans, critical functions of the maternal and paternal genomes are derived from the striking differences in epigenetic modification by DNA methylation between oocytes and sperms. This phenomenon of epigenetic modification is called genomic imprinting. Parental-origin-specific monoallelic gene expression of imprinted genes is regulated by DNA methylation in the differentially methylated region (DMR), and epigenetic modification is independently imposed during oogenesis and spermatogenesis. The majority of imprinted genes are believed to be epigenetically modified during oogenesis. In this paper, I review the DNA methylation imprints during mouse oogenesis.