The LiMnPO4 cathode material is a good candidate for rechargeable lithium ion batteries because of high voltage (4.1 V), high thermal stability and no toxicity. However, the rate performance of this material is extremely low when compared to the conventional LiMO2 (M = Ni or Co) or LiFePO4. In this study, the primary particle size was reduced by a hydrothermal synthesis, and we obtained LiMnPO4 particles with sizes less than 40 nm. The sample exhibited an excellent rate capacity, which was 136 mAh g−1 at 5 C and 120 mAh g−1 at 10 C. We also revealed the effect of the LiMnPO4 particle size on the battery by introducing STEM-EELS, which was expected to allow for the direct microscopic observation of the lithium state in the particles during the charge-discharge process. With the EELS analysis, it was clarified that the lithium diffusion distance from the inside to the surface of the primary particles had to be reduced down to about 20 nm to achieve the high rate performance.