Lithium iron phosphate (LFP) was prepared by hydrothermal synthesis in an organic-free system. Samples were collected at different time spots during temperature rising and soaking period and characterized by X-ray diffraction, scanning electron microscopy, B.E.T. specific surface area and particle size analysis. Two precursors, Li3PO4 and Fe3(PO4)2·8H2O, form easily at ambient temperature. They dissolve into ions once temperature increases to 135°C. LFP nuclei form rapidly at 135–140°C and grow fast at 140–150°C. The crystal morphology transforms from diamond to polygon plate at 150–160°C. The B.E.T. specific surface area and particle size decrease as reaction time prolongs. The thickness of the platelets remains unchanged within the range of 60–100nm for each LFP sample. The sample synthesized at 160°C for 2h and then coated with carbon (LFP/C-2) exhibits optimum electrochemical performances that the specific discharge capacities are 162.1, 150.8 and 136.9mAhg−1 at 0.2C, 1C and 5C, respectively. Reducing the precursors’ particle size would be an effective way to shorten the reaction time and obtain fine particles with high specific surface area and high purity.