The thermal stability of the main constructive materials for Li(Ni_xCo_yMn_z)O₂/Li₄Ti₅O₁₂ batteries was evaluated using a C80 micro calorimeter. The thermal decomposition of anode and cathode, the heat of reactions of electrolyte with anode and cathode, and the heat of reactions of an integral cell were characterized. It follows that with rising temperature the system of anode Li(Ni_xCo_yMn_z)O₂/electrolyte undergoes two exothermic processes with a total heat generation of -526.0 Jg^-1 and activation energy of 273.8 kJmol^-1; while the system of Li₄Ti₅O₁₂/electrolyte undergoes four exothermic processes with a total heat generation of -291.5 Jg^-1 and activation energy of 61.8 kJmol^-1; the reaction processes for an integral cell are the overlap of the reaction processes occurred in the two half cells Li(Ni_xCo_yMn_z)O₂/electrolyte and Li₄Ti₅O₁₂/electrolyte. The heat runaway phenomenon is triggered by the reactions of the anode/electrolyte, while, of which the main heat source may come from the reactions of the cathode/electrolyte.