Coating LiMnPO4 with a thin layer of LiFePO4 shows a better electrochemical performance than the pure LiFePO4 and LiMnPO4, thus it is critical to understand Li diffusion at their interfaces to improve the performance of electrode materials. Li diffusion at the (100)LiFePO4//(100)LiMnPO4, (010)LiFePO4//(010)LiMnPO4, and (001)LiFePO4//(001)LiMnPO4 interfaces between LiFePO4 and LiMnPO4 was investigated using density functional theory. The calculated diffusion energy barriers are 0.55 eV for Li to diffuse along the (001) interface, 0.44 and 0.49 eV for the Li diffusion inside the LiMnPO4 and along the (100) interface, respectively. When the Li diffuses from the LiFePO4 to LiMnPO4 by passing through the (010) interfaces, the diffusion barriers are 0.45 and 0.60 eV for the Li diffusions in both sides. The diffusion barriers for the Li to diffuse in LiMnPO4 near the interfaces decrease compared with those in the pure LiMnPO4. The calculated diffusion coefficient of Li along the (100) interface is in the range of 3.65×10-11-5.28×10-12 cm2/s, which is larger than that in the pure LiMnPO4 with a value of 7.5×10-14 cm2/s. Therefore, the charging/discharging rate performance of the LiMnPO4 can be improved by surface coating with the LiFePO4.