Lithium iron phosphate particles were synthesized with supercritical water as a reaction medium, and the effects of temperature and pressure on particle purity and size were examined. A flow-through reactor was designed and set up with a SUS316L stainless steel material, and a large yield could be obtained with no preheated water involved. The products were recovered and characterized by X-Ray Diffration (XRD), Laser Particle Analyzer (LPA), and Scanning Electron Microscopy (SEM). Particles with an average size of 500nm could be obtained continuously.