High-voltage and high-capacity cathode-active materials are required to increase the energy density of rechargeable lithium-ion batteries for hybrid vehicles. The olivine-type LiMnPO4 is considered as a good candidate for the next-generation lithium-ion battery due to its high voltage (4.1 V vs Li+/Li), low cost, and lower toxicity compared with the currently used layered materials. However, recent research has demonstrated that the thermal stability of delithiated phase of Li1−xMnPO4 (0 ≤ x <1) was less than that of Li1−xFePO4. These reports verified that the delithiated MnPO4 decomposed and changed into Mn2P2O7 with O2 release at high temperature. In this study, we focused on the particle and crystal changes in LiMnP O4/MnPO4 at high temperature on a nanoscale. As a result, we have succeeded to observe directly the particle and crystal changes using transmission electron microscope (TEM) with heating. It revealed that MnPO4 was a thermally unstable phase because dendrites of Mn2P2O7 began to generate around 200°C.