The micron-sized LiMn0.8Fe0.2PO4 (LMFP) mesocrystals with high volumetric density were successfully synthesized via a solvothermal method. The LMFP mesocrystals, composed of ∼30-nm-sized nanocrystallites, exhibit a high tap density of ∼1.2g/cm3. The synthesis of such nano-sized crystals was rendered by ascorbic acid which serves as surface-energy modifier. Interestingly, the ascorbic acid also directed the primary nanocrystals to anisotropic aggregations, and the growth mechanisms were rationally identified by TEM and x-ray diffraction. For reversible Li insertion/extraction, the interconnected particles within the mesocrystals were coated by conductive carbon, and the enhancement of the electrochemical properties (i.e. high specific capacity and good rate capability) was assessed by galvanostatic cycling. We believe that this work provides one of the routes to design electrochemically-favorable meso/nano-structures, which is of great potential for improving the battery performance by tuning the morphology of particles at the multi-length scale.