Multi-stage ignition and/or double NTC (negative temperature coefficient) behavior resulted from the low-temperature oxidation of ether compounds are still not clearly explained. We have investigated the oxidation mechanism of a stoichiometric DEE (diethyl ether)/air mixture by using a micro flow reactor with a controlled temperature profile to see the detail of low-temperature weak flame structure. The simulation was also performed to understand the chemical kinetics mechanism of observed weak flame structure. Chemiluminescence measurement showed separated weak flame in the temperature range of 600 K–800 K. The simulation also qualitatively reproduced this separated weak flame, and showed four peak of heat release. From the reaction flow analysis, it was found that (1) O-O bond scission reaction of keto-hydroperoxide produced by DEE, (2) O-O bond scission reaction of CH3O2H, CH3CO3H, and C2H5O2H, (3) O-O bond scission reaction of H2O2, and (4) H+O2=O+OH are key chain branching reactions to explain the multi-stage oxidation.