The phase stability of Li1-xMnPO4 (0 <= x <= 1) is investigated in this study for different Li compositions and temperatures by high temperature X-ray diffraction and electron microscopy. The map of stable phases is determined at temperature ranges between room temperature and 410 degrees C. While pure LiMnPO4 phase is stable at high temperature, partial phase transformation of MnPO4 into Mn2P2O7 is observed in delithiated phases above 210 degrees C. Electron microscopy study also indicates the instability of the delithiated phase. The morphology of LiMnPO4 is severely damaged upon delithiation. The instability of the delithiated phase and the phase transformation into Mn2P2O7 may imply that safety concerns can be raised regarding the LiMnPO4 cathode, unlike its Fe counterpart.