Lithium-ion batteries are the main energy storage unit for electric vehicles. The prevention of thermal runaway is essential for ensuring safe operation of these batteries. Different cell packaging patterns have an influence on the thermal runaway behavior of lithium-ion batteries during overcharging. In this paper, prismatic and pouch lithium-ion battery cells with the same capacity and chemistries are used to experimentally investigate the internal failure mechanisms and associated external characteristics during overcharging. The entire overcharge process is divided into five stages according to the voltage and temperature curves. The results indicate that the pouch battery cell exhibits better thermal behavior characteristic and overcharge tolerance when compared to the prismatic battery cell in stages I to III. However, the prismatic battery cell has better thermal runaway buffering characteristic, smaller deformation and longer early warning time. This is due to the use of a safety valve which results in subsided damage caused by thermal runaway. Their maximum surface temperature differences increase linearly with the overcharge process, until thermal runaway occurs and rapid rise to the highest temperature point takes place. These results provide an insight into the safety and thermal management design of battery systems.