Ionic liquids (ILs) may have various applications as less hazardous solvents because of their thermal stabilities and low vapor pressures. However, it is challenging to remove heat from large amount because of IL's high viscosity and low vapor pressure when cooling failure or abnormal reaction occurs. Because chemical processes using ILs on an industrial scale are still in the early phase of development, it is important to assess the associated thermal hazards of chemical process using ILs. This work proposes a systematic approach of analyzing thermal hazards of chemical process using ILs, based on the approach proposed by Stoessel and analyze the cellulose dissolution process using ILs containing impurities as a test case. Copper oxide, which might be contaminated from wood chips, was chosen as a model impurity. To analyze the thermal hazards of cellulose dissolution process using ILs, thermal analysis using small-scale reaction calorimeter, differential scanning calorimeter and thermogravimetry-differential thermal analysis-mass spectrometry were performed. From these results, reaction heat and heat value of ILs and its mixture were low, while CuO lowers the decomposition temperature of ILs and produces flammable gases. From these trials, exothermal reaction was not observed, while flammable gaseous products might be produced in vapor phase in the reactor.