Abstract: To research epigenetic regulation of genome of Fraxinus mandshurica seed development ripening period, Fraxinus mandshurica cotyledons from early July to mid-September 8 stages were selected as test material. Through MSAP(methylation-sensitive amplified polymorphism)method, we detected the change of DNA ethylation in each period. The result indicated that no significant difference among different development stages was detected(in total 933-939 clearly identified strips was detected in each stage). However, the cytosine methylation patterns and variation pattern of CCGG sites showed that, in early July, CNG, CG/CNG loci demethylation sites were higher than other stage, which indicated that large scale of DNA demethylation occurred following seed development of Fraxinus mandshurica. This result could serve as the regulation patterns and as an important basis for providing theoretical basis for seed development.