Olivine-type lithium manganese phosphate (LiMnPO4) has been considered as a promising cathode for next-generation Li-ion batteries. Preparation of high-performance LiMnPO4 still remains a great challenge because of its intrinsically low Li-ion/electronic conductivity. In this work, significant performance enhancement of LiMnPO4 has been realized by a controllable acid-engaged morphology tailoring from large spindles into small plates. We find that acidity plays a critical role in altering the morphology of the LiMnPO4 crystals. We also find that size decrease and plate-like morphology are beneficial for the performance improvement of LiMnPO4. Among the plate-like samples, the one with the smallest size shows the best electrochemical performance. After carbon coating, it can deliver high discharge capacities of 104.0 mAh g(-1) at 10 C and 85.0 mAh g(-1) at 20 C. After 200 cycles at 1 C, it can still maintain a high discharge capacity of 106.4 mAh g(-1), showing attractive applications in high-power and high-energy Li-ion batteries.