Microwave irradiation assisted synthesis (MIAS) is successfully employed to prepare three-dimensional (3D) porous microspherical aggregation of LiFePO4/C nanoparticles as the cathode material of lithium ion battery. XRD, SEM, and TEM studies confirm that highly crystalline LiFePO4 without any impurity phase can be synthesized under microwave irradiation of 700 W for 5 min. The as-obtained LiFePO4/C sample is prepared via MIAS consisting of nanopores (20–30 nm) and nanoparticles (20–30 nm) which are coated with uniform, complete carbon layers (about ∼3 nm). These superior physical characters provide LiFePO4/C (MIAS) excellent electrochemical performance at 0.1C including the high discharge capacity of 156.9 mAh g−1 and the low polarization of 51.5 mV. At the discharge rate of 1C, LiFePO4/C (MIAS) exhibits the low polarization of 202.3 mV, high capacity of 126.7 mAh g−1, high discharge voltage of 3.33 V, and specific energy of 398.5 kWh kg−1, all of which are much better than those measured for the sample prepared via conventional heat treatment (CHT). At even high rate of 5C, LiFePO4/C (CHT) fails to discharge whereas LiFePO4/C (MIAS) is able to deliver the discharge capacity of 95.5 mAh g−1. The low polarization of LiFePO4/C (MIAS) results in the good discharge voltage of 3.06 V, high specific energy of 278.8 Wh kg−1, and excellent energy efficiency of 72.0% at 5C.