Among the alternative fuels, dimethyl ether (DME), one of the oxygenated fuels, attracts attention as an alternative fuel for the Diesel engine since the properties of the DME are fitted to the Diesel engine combustion and the know-how development has been made of the mass production of the DME from a natural gas. In this study, experiments were performed of ignition characteristics of the DME and Diesel fuel sprays injected by a D.I. Diesel injector into a high-pressure, high-temperature vessel. The fuel injection was made by a Bosch type injection system. A schlieren optical system was adopted for visualizing the ignition process as well as the vaporization process of the DME and Diesel fuel sprays. The ignition delay was measured by using a photo-sensor which had a sensitivity in the wavelength range from visible to ultraviolet. Pressure and temperature of the ambient air and the oxygen concentration of the ambient air were changed as experimental parameters. In the case of the ordinary air, the ignition delays of the DME and Diesel fuel sprays under the high ambient pressure are almost the same. Both ignition delays show the similar variation with the ambient temperature. Under the low ambient pressure, the ignition delay of the DME spray is shorter than that of the Diesel fuel spray. When the ambient air is changed from the ordinary air to the low oxygen concentration air, an increase in the ignition delay is much smaller for the DME spray than that for the Diesel fuel spray.