Morphology-controlled monodispersed LiMnPO₄ nanocrystals as high-performance cathode materials for Li-ion batteries have been successfully synthesized by a solvothermal method in a mixed solvent of water and polyethylene glycol (PEG). Morphology evolution of LiMnPO₄ nanoparticles from a nanorod to a thick nanoplate (∼50 nm in thickness) and to a smaller thin nanoplate (20–30 nm in thickness) is observed by increasing the pH value of the reaction suspension. Electrochemical measurements confirm that the LiMnPO₄ thin nanoplates display the best charge–discharge performance, thick nanoplates the intermediate, nanorods the worst, which can be mainly ascribed to the difference in their morphologies and particle sizes in three dimensions. Further modification of LiMnPO₄ thin nanoplates with graphene gives rise to an improved electrochemical performance compared with conventional pyrolytic carbon coated ones. The LiMnPO₄ thin nanoplate/graphene composites deliver a high capacity of 149 mA h g⁻¹ at 0.1 C, 90 mA h g⁻¹ at 1 C, and even 64 mA h g⁻¹ at 5 C charge–discharge rate, with an excellent cycling stability.