The mass ratio between electrode and electrolyte in lithium-ion battery plays a key role for the battery thermal stability. Its effect on the thermal stability of their coexisting system was studied using C80 micro-calorimeter. For the Li0.5CoO2–LiPF6/ethylene carbonate (EC)+diethyl carbonate (DEC) coexisting system, when the mass ratios of Li0.5CoO2–LiPF6/EC+DEC are 2:1, 1:1, 1:2 and 1:3, one, two, three and four main exothermic peaks are detected with total heat generation of −1043.8Jg−1, −1052.6Jg−1, −1178.5Jg−1 and −1684.5Jg−1, respectively. For the Li x C6–LiPF6/EC+DEC coexisting system, the thermal behavior trend is similar, and the heat generation increases with the electrolyte content increasing, however, and the onset temperature are very close to each others. The heating rate also influence the heat generation rate for the two coexisting system, too far or too low heating rate could results in varies heat generation.