Exhaust emission characteristics of direct injection diesel engines under dual-fuel operations with dimethyl ether (DME) were investigated. Two types of DME mixing methods were employed; one is fumigation and the other is binary-fuel injection (BFI). When the DME was used with the fumigation method, low emissions in THC and high emissions in smoke were obtained in comparison with diesel fuel alone. In the BFI method, DME mixing caused a decrease in the NO emissions without increasing the THC emissions, but some amount of smoke was exhausted at high load. Knocking cycles occurred for over 60 % DME fraction.