This study was conducted to investigate the effect of ambient conditions on the spray characteristics of DME fuel injected from a common-rail injection system. The spray characteristic of DME fuel was analyzed in terms of spray tip penetration, spray cone angle, impingement timing and centroid of spray. For the illuminated of spray, the spray visualization system was composed of a Nd:YAG laser and an ICCD camera and laser-sheet method was used. A high pressure chamber with optical window was used to allow the simultaneous observation of the effect of ambient conditions on the spray characteristics of DME fuel. The experiments are conducted in variable injection, ambient, temperature and combustion geometry conditions. It is revealed that DME fuel shows a shorter spray path penetration, wider spray cone angle and much quicker evaporating process than diesel fuel. The spray area of DME is increased with increase of ambient temperature and centroid of spray is move to the downstream of spray development. In the case of non-evaporating spray characteristics, the spray development of DME is slower and rapidly disappeared as elapsed time after start of injection at the same injection duration. The impingement timing of diesel fuel was faster than that of DME fuel.