A series of LiMn0.9Fe0.1−x Mg x PO4/C (x =0, 0.01, 0.02, 0.05) was synthesized by a solid state reaction, and the effect of synthesis temperature and Fe/Mg ratio on the electrochemical performance of the obtained materials was investigated by X-ray diffraction, scanning electron microscopy, Raman spectroscopy and electrochemical measurements. The electrochemical performance of the Fe and Mg co-substituted LiMnPO4 was obviously improved with increasing synthesis temperature from 650 to 800°C, but further increase led to an abrupt capacity loss due to the impurity formation. The Fe and Mg co-substitution could remarkably enhance the electrochemical activity of LiMnPO4 compared with the Fe substitution only, but too high level of Mg doping would worsen the rate capability. The LiMn0.9Fe0.09Mg0.01PO4/C synthesized at 800°C demonstrated the optimum electrochemical performance with a high capacity and an excellent rate capability. Even discharged at the rate of 10C, a capacity of 60mAhg−1 was still observed.