Rate equation of LiFePO4 formation via solid-state reaction has been studied by using a model system: Li(CH3COO)+FePO4 in a reducing atmosphere. Kinetic data were acquired by using in-situ synchrotron X-ray diffraction technique, and the analysis was based on a non-isothermal methodology, which shows that the reaction rate is well described by the rate-equation: [F 0.7/(1− F)0.7]=1.56×1011 exp(−24,100/T)t, where F is the fractional conversion to LiFePO4, T, the calcination temperature (K), and t is the calcination time (min). The equation indicates that the formation of LiFePO4 is intrinsically a fast reaction: 95% conversion can be achieved between 550 and 600°C in a few hours. Nevertheless, the reaction could be significantly hindered if gas-phase diffusion processes of reactant/product species become rate-limiting, and the gas-flow pattern relative to the powder bed during synthesis thus has a decisive effect on the reaction rate in large-scale synthesis. Single-phased, nanocrystalline LiFePO4 powder having an average crystal size of 35nm can be synthesized by calcination using flow-through configuration at 600°C in merely 2h, and the powder exhibits a capacity of ∼140mAhg−1.