Epigenetic modification is the main mechanism of transcriptional regulation that does not involve changes in DNA sequences, such as DNA methylation, acetylation and methylation of the N-terminal tail of histone. Recently, next-generation sequencing technology has provided detailed information about the DNA methylation status of the whole mouse genome in full-grown oocytes. However, it is still very hard to read histone codes in oocytes because a large number of cells (1 × 10⁶ cells or more) are needed for such analyses. In addition, information that can be obtained from immunostaining analysis is limited to a global image of histone modification in oocytes. Consequently, a complete picture of individual epigenetic modifications in mouse oocytes has not yet been understood. In this paper, the DNA methylation required for functional oocytes is reviewed. The differences in DNA methylation between oocytes grown in vivo and in vitro, and the potential for manipulating epigenetic modifications in oocytes are also discussed.