In mammals, some genes categorized as imprinted genes are exclusively expressed either from maternal or paternal allele. This parental-origin-specific gene expression is regulated by epigenetic modification of DNA methylation in differentially methylated region (DMR), which is independently imposed during oogenesis and spermatogenesis. It is known that methylation of DMR in the female germ line is established during oocyte growth phase. However, the cause of the progression of methylation on DMR, due to either aging of mice or growth-size of oocyte was unclear up to now. Here, we analyzed the methylation of DMR for each eight imprinted genes (Igf2r, Lit1, Zac1, Snrpn, Peg1/Mest, Impact, Meg1/Grb10, and H19) by bisulfite sequencing methylation assay, using oocytes from 10 dpp (days post partum), 15 dpp, 20 dpp, and adult mice. To find whether the size of oocytes is the cause of methylation, above oocytes were classified into seven groups (each oocyte diameter ranging from 40 to 75 microm with intervals of 5 microm). The results from juvenile mice oocytes showed that DMR methylation progressed according to oocyte growth each imprinted gene. More than 85% of DMR methylation was achieved for both Igf2r, Zac1 & Lit1 with oocyte size of reaching 55 microm and Snrpn, Peg1/Mest, Impact, and Meg1/Grb10 with oocyte size of reaching 60 microm. Preferential methylation of maternal allele was observed in Zac1 and Peg1/Mest of juvenile oocytes and in Snrpn of juvenile and adult oocytes. The oocyte size-dependent-methylation progressed equally for all three different-age juvenile mice. The size-dependent-methylation was also recognized in the growing oocytes collected from adult mice, although the progress is slightly slower than that of juvenile mice. From these results, we concluded that DNA methylation is established with oocyte size dependent manner, not with aging of mice.