This work is to investigate the macroscopic spray characteristics and internal flow pattern of dimethyl ether (DME) discharged through twin-orifice injectors. Transparent injectors were manufactured to study the effects of different diameters of discharge nozzles on the internal flow and atomization. Injection pressures of 4.0 MPa and 2.0 MPa were implemented to produce "stable" flash-boiling sprays vertically, and the ambient condition was atmospheric. Internal flow in the expansion chamber (EC) and external liquid atomization were observed simultaneously during the actuation. The relationship between the internal boiling flow in the EC and external flash-boiling spray were analyzed. Lower gas/liquid mass ratio in the EC generated larger external spray angles and lead to a sudden drop of spray angles when the supply valve closed. However, higher gas/liquid mass ratio (homogeneous bubbly flow) in the EC prolonged the stability of sprays when the solenoid valve was shutting down, but with smaller spray angles. The effect of expansion chamber volume was obvious at lower injection pressure. It is suggested to reduce the EC volume when the injection pressure is relatively low. This work provides a guide and reference for the port-fuel injection of DME engines. (C) 2016 Elsevier Ltd. All rights reserved.