Ammonium diniramide (ADN)-based energetic ionic liquids (EILs) are promising new liquid rocket propellants for small satellites because of their high energy content and safety. This research focused on a mixture of ADN and hydrazide compounds. When simply mixed at room temperature, two solids' 1:1 (mass ratio), ADN (melting point = 92°C), and formic hydrazide (melting point = 56°C) formed a eutectic mixture. Additionally, they ignited at a lower temperature than other ADN-based EILs when a droplet was dropped into a heating furnace. To better understand the thermal decomposition mechanism in the condensed phase, the thermal behavior, and evolved gas during constant rate heating were examined using sealed cell differential scanning calorimetry (SC-DSC) and thermogravimetry-differential thermal analysis-mass spectrometry. Although in SC-DSC, the onset temperature of ADN/formic hydrazide's exothermic reaction was almost the same as that of pure ADN, heat quantity was higher than that of pure ADN. The evolved gas of ADN/formic hydrazide's decomposition seemed to contain a small amount of hydrazine. The ADN/formic hydrazide's low ignition temperature will be realized due to the reaction between hydrazine and nitrogen dioxide or nitric acid formed from the condensed phase reaction.