The B3LYP density functional theory (DFT) was used to investigate the ignition mechanism for the hypergolic reaction of DMAZ with pure nitric acid (HNO3). Results identify two controlling reaction pathways that are both exothermically and kinetically favored. The first reaction is the proton transfer reaction from HNO3 to the amine nitrogen of DMAZ in the liquid phase. The second is the gas-phase reaction between DMAZ and NO2, which is the product from the thermal decomposition of HNO3. Compared with other non-azido hypergolic propellants, the azido group of DMAZ is found to play an important role in suppressing the proton transfer to the amine nitrogen and hence reducing the reactivity of DMAZ to be a hypergolic fuel. The results agree with recent experimental observations on the ignition delay of DMAZ with nitric acid.