There are three members in DNA methyltransferase family of mammals, DNA methyltransferase 1 (Dnmt1), DNA methyltransferase 2 (Dnmt2, also named tRNA aspartic acid methyltransferase 1, TRDMT1) and DNA methyltransferase 3 (Dnmt3). Dnmt2 is immemorial than Dnmt1 and Dnmt3, and it exists in plant, fungi, parasites, insects, fish, amphibians, aves and mammalia, however, its biological function is still unknown. The structure characteristics, RNA methylation, function analysis and scientific problems are discussed in this paper.