Recently, micron-sized LiMn 1−x Fe x PO 4 cathode materials have attracted attention due to its better rate capability and higher tap density than the nano-sized ones. However, the influence of the particle size on the energy density of micron-sized LiMn 1−x Fe x PO 4 is still unknown. In this paper, we report the optimal particle size of the micron-sized LiMn 0.8 Fe 0.2 PO 4 with enhanced electrochemical performance as cathode material in lithium-ion batteries (LIBs). The LiMn 0.8 Fe 0.2 PO 4 sample with the particle size of ∼9.39 μm delivers the initial discharge capacity of 124 mAh g −1 at 0.2 C rate with high capacity retention of 94.35% after 100 cycles, which is higher than that with the particle sizes of ∼2.71 μm, ∼3.74 μm, ∼6.41 μm or ∼16.31 μm. This structure with the specific capacity of 122 mAh g −1 at 0.5 C rate and 106 mAh g −1 at 3 C rate also exhibits excellent rate performances. The improved electrochemical performances are mainly derived from its fast Li + diffusion, which causes the higher ionic conductivity. The LiMn 0.8 Fe 0.2 PO 4 sample with the particle sizes of ∼9.39 μm also shows the highest tap density (0.68 g cc −1 ) among the as-prepared samples. This finding provides a new way to enhance the energy density of other cathode materials.