Although gene duplication plays a major role in organismal evolution, it may also lead to gene dosage imbalance, thereby having an immediate adverse effect on an organism's fitness. Investigating the evolution of the expression patterns of genes that duplicated after the divergence of rodents and primates, we confirm that adaptive evolution has been involved in dosage rebalance after gene duplication. To understand mechanisms underlying this process, we examined 1) microRNA (miRNA)-mediated gene regulation, 2) cis-regulatory sequence modifications, and 3) DNA methylation. Neither miRNA-mediated regulation nor cis-regulatory changes was found to be associated with expression reduction of duplicate genes. By contrast, duplicate genes, especially lowly expressed copies, were heavily methylated in the upstream region. However, for duplicate genes encoding proteins that are members of macromolecular complexes, heavy methylation in the genic region was not consistently observed. This result held after controlling potential confounding factors, such as enrichment in functional categories. Our results suggest that during mammalian evolution, DNA methylation plays a dominant role in dosage rebalance after gene duplication by inhibiting transcription initiation of duplicate genes.