This is a part of our Li-diffusion study in battery materials with muSR. In order for balancing safety and energy density, a solid solution system between LiFePO4 and LiMnPO4 has been heavily investigated as a cathode material of an advanced Li-ion battery, because a charged state LiFePO4, i.e. Li1-yFePO4 is very stable even at 150 C and LiMnPO4 has a higher working voltage. Although the diffusion coefficient of Li (D) in LiFePO4 was successfully measured with muSR, it was very difficult for LiMnPO4 due to large Mn moments. However, if we measure D with muSR as a function of x in LiFe1-xMnxPO4, we will extrapolate D for LiMnPO4. Here, D is a very important parameter to determine the total performance of battery and estimate other significant electrochemical properties, such as, a reactive surface area and a concentration of mobile Li.