Lithium iron phosphate (LiFePO₄) was synthesized by a hydrothermal method at 120, 140, 150, 160 and 175 °C from LiOH, FeSO₄ and H₃PO₄ in an organic-free aqueous solution. The intermediates of the product synthesized at 175 °C were sampled at 120, 130 and 135 °C during the temperature rising period. The Rietveld refinement results of X-ray powder diffraction (XRD) show that the unit cell volume of LiFePO₄ prepared at 120 °C is 0.2915 nm³, which decreases to 0.2900 ± 0.0001 nm³ as the reaction temperature reaches above 140 °C. The results from scanning electron microscopy (SEM) show that the crystal morphology develops from a diamond plate to a polygonal plate and the thickness of the flakelet decreases from 130–150 nm to 80–90 nm with an increasing temperature from 120 to 175 °C. A critical temperature, ca. 135 °C, for the rapid nucleation is found from the XRD results of the intermediates. The variations in crystallinity, morphology and size between the samples prepared under and above 140 °C are due to the varied rates of nucleation and crystal growth of LFP particles, which are dependent on the concentration of Fe²⁺ and PO₄³⁻ resulting from the dissociation of the intermediate Fe₃(PO₄)₂·8H₂O. The sample prepared at 160 °C exhibits specific discharge capacity of 161.2 mAh g⁻¹ at 25 °C and 121.6 mAh g⁻¹ at −20 °C measured at a current density of 17 mA g⁻¹.