Plate-like LiMnPO4 nanomaterial for Li-ion batteries is synthesised successfully via a facile solvothermal process in mixed water-diethylene glycol (DEG) solvents at 190 °C for 3 h. Experimental variables, including reaction time, reaction temperature and reactant mole ratio, are discussed in detail. A tentative reaction mechanism is proposed on the basis of the time dependent trials. It is found that, with the increase of reaction temperature, the formation of LiMnPO4 phase is accelerated and the reaction for synthesizing LiMnPO4 can be finished in a shorter time. Studies on the effect of reactant mole ratio further support the proposed mechanism. The electrochemical properties of obtained LiMnPO4 are examined after a carbon coating process. Electrochemical tests show that the obtained LiMnPO4 at 190 °C for 3 h exhibits better electrochemical performances than the LiMnPO4 synthesized at 170 °C for 4.5 h or 150 °C for 6 h. It is clear that an improved electrochemical performance can be obtained with the increase of reaction temperature and the decrease of reaction time. This result provides us thoughts and guidance to optimize the reaction conditions by harmonizing the reaction temperature and reaction time, which is beneficial for the practical application.