The thermal runaway behavior of lithium-ion cells plays a crucial role in the safety management of the powertrain in electric vehicles. In this study, the effect of states of charge (SOC) on the thermal runaway behavior of commercial LiNi1/3Mn1/3Co1/3O2 (NMC)-based pouch cells is investigated using accelerating rate calorimetry (ARC) and ex-situ X-ray diffraction. By studying the differences in the onset temperature of self-heating (T1) and the onset temperature of thermal runaway (T2) along with the mass loss between the different SOCs, we observed that higher SOC led to a decrease in the T2. However, T1 initially increased and then decreased with increasing SOC. These trends were attributed to the phase change of cathode material and separator. The ARC results also indicated the occurrence of endothermic reactions during the self-heating accumulation period. The findings in this study are helpful for thermal safety management of battery powertrain for electric vehicles.