Dimethyl ether (DME) is a promising alternative fuel for compression ignition (CI) engines. DME features good auto ignition characteristics and soot-free combustion. In order to develop an injection system suitable for DME, it is necessary to understand its fuel properties. Sound speed is an important fuel property that affects the injection characteristics. However, the measurement data under high-pressures corresponding to those in fuel injection systems are lacking. The critical temperature of DME is lower than that of diesel fuel, and is close to the injection condition. It is important to understand the behavior of the sound speed around the critical point, since the sound speed at critical point is extremely low.In this study, sound speed in DME in a wide pressure and temperature range of 1 MPa to 80 MPa, 298.15 K to 413.15 K, including the vicinity of the critical point, was measured. The sound speed in DME decreases as either the pressure falls or the temperature rises. It is approximately 400 m/s slower than that found in diesel fuel. Around the critical point, the sound speed drastically decreases due to a lowering of bulk modulus. At a low-pressure region below 20 MPa including the vicinity of the critical point, the experimental results were consistent with the calculated results based on Helmholtz equation of state, proposed by Wu et al. . The calculated value in the high-pressure range has a tendency to register a slightly higher than the experimental one as the pressure rises.