FePO4 nanoparticles, acted directly as cathode materials in lithium ion rechargeable batteries, were synthesized by a microemulsion technique. The various grain-sized and crystal-structured FePO4 samples were obtained by sintering at different temperatures (380°C, 460°C, 550°C, 650°C) for 3h in air. The structure and morphology were investigated by means of X-ray diffraction (XRD) and field emission-scanning electron microscopy (FE-SEM); the electrochemical properties were characterized by electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and galvanostatic charge and discharge tests. Results show that FePO4 sintered at 380°C and 460°C for 3h are amorphous, and with fine sizes in the range of 10–20nm. Increasing the sintering temperature leads to an increase in grain size and makes the structure change from amorphous to trigonal. The EIS results show that the R ct value of FePO4 sintered at 460°C is smaller than that sintered at 650°C. The diffusion coefficients of lithium ion (D Li) of FePO4 sintered at 380°C, 460°C, 550°C and 650°C measured by EIS are 8.09×10−14, 1.06×10−13, 4.88×10−14 and 2.59×10−14 cm2 s−1, respectively. The difference in diffusivity is also confirmed by CV and the D Li values are 6.71×10−14, 8.28×10−14, 4.89×10−14 and 1.60×10−14 cm2 s−1, respectively. The correlations between the electrochemical performances of FePO4 and lithium ion diffusion are acquired. The charge and discharge tests show that the initial discharge specific capacity of FePO4 sintered at 460°C for 3h reaches 142.3mAhg−1 at 0.1°C. These results suggest that nanoparticles and amorphous FePO4 facilitates lithium ion diffusion during the charge/discharge cycles.