Carbon-coated clustered LiMn0.71Fe0.29PO4 (c-LMFP) nanoparticles are prepared from ball-milling with a mixture of ∼40 nm thick LMFP nanoplates obtained by polyol method and carbon black. The clustered nanocomposite structure of c-LMFP turns out to have advantages of improved volumetric energy density and electrochemical performance. The c-LMFP exhibits increased tap density of 0.9 g cm−3, compared with the as-prepared LMFP nanoplates (0.6 g cm−3), providing with high volumetric discharge capacity of 243 mA h cm−3 at 0.1C and 128 mA h cm−3 even at 7C at 21 °C. At elevated temperature (60 °C), the capacity retention of c-LMFP remains excellent (100% of its initial capacity (165 mA h g−1) at the same cycling condition as 21 °C). In sharp contrast, capacity of carbon-coated LiMnPO4 (c-LMP) exhibits volumetric discharge capacity of 72 mA h cm−3 at 5C and decays rapidly at 60 °C after 40 cycles (capacity retention of 58%). The better cycling stability of c-LMFP than that of c-LMP is believed to be associated with mitigated Mn2+ dissolution by Fe2+ substitution.