Genomic imprinting describes the epigenetic phenomenon is that the parent's origin is marked on genome. The phenomenon is unique to placental mammals, and causes parent-of-origin-specific monoallelic expression of most of imprinted genes. The previous studies on mice clarified that these gene expression control required the sex-specific differences of DNA methylation which were established in oogenesis or spermatogenesis. In this paper, we review the relevance of genomic imprinting mechanism and DNA methylation.