Epigenetic regulations including DNA methylation, long noncoding RNAs and histone modification are considered to be involved in many biological processes. Such regulations in general begin with change of covalent bonds on the substrates. Moieties involving the covalent bond include methyl- and acetyl-group, glucose, SUMO and etc. Among them, methyl group-mediated modulation is commonly observed in all three substrates. Mouse primordial germ cells (PGCs) first appear at embryonic day (E)7.25 on the base of the allantois, and then migrate through the hindgut to the genital ridge. Once PGCs reach genital ridge, they become dimorphic, in that female PGCs undergo meiosis whereas male PGCs are mitotically arrested. Meiosis is a germ-cell-specific cell division process through which haploid gametes are produced for sexual reproduction. Before the initiation of meiosis, mouse primordial germ cells undergo a series of epigenetic reprogramming steps including the global erasure of DNA methylation at the 5-position of cytosine (5mC) in CpG-rich DNA. I will discuss role of Ten-to-Eleven translocation (Tet) in DNA demethylation in the process of PGC reprogramming.