Recognition of Dimethyl Ether (DME) as an alternative fuel has been growing recently due to its fast evaporation and ignition in application of compression-ignition engine. Most importantly, combustion of DME produces almost no particulate matter (PM). The current study provides a further understanding of the combustion process in DME reacting spray via experiment done in a constant volume combustion chamber. Formaldehyde (CH2O), an important intermediate species in hydrocarbon combustion, has received much attention in research due to its unique contribution in chemical pathway that leads to the combustion and emission of fuels. Studies in other literature considered CH2O as a marker for UHC species since it is formed prior to diffusion flame. In this study, the formation of CH2O was highlighted both temporally and spatially through planar laser induced fluorescence (PLIF) imaging at wavelength of 355-nm of an Nd:YAG laser at various time after start of injection (ASOI). The injection pressure was kept at 750 bar using a single-hole injector with diameter of 180 μm. The formation of CH2O is also correlated with time of ignition and flame region using other optical diagnostics including photodiode and natural flame luminosity, respectively. Additionally, kinetic modeling and CFD were used in order to explain further and confirm the combustion process and species formation (particularly, CH2O) in the DME flame. Both experimental and numerical results showed that CH2O is formed mostly prior to flame ignition at upstream region before liftoff length.