In this work, the size adjustment of FePO4 nanoparticles in the range of 9-50 nm was conducted on a general platform of coupling fast precipitation in a microreactor and thermal treatment process. Specifically, we used the mixture of nitric acid and phosphoric acid as the continuous fluid to change the free Fe3+ concentration and control the supersaturation of FePO4 in fast precipitation. Inductively coupled plasma optical emission spectroscopy (ICP-OES) and X-ray diffraction (XRD) verified that as-prepared nanoparticles were high-purity amorphous FePO4 center dot 2H(2)O. Transmission electron microscopy (TEM) verified their good dispersity and narrow size-distribution (standard deviation, similar to 5 nm). Brunauer-Emmet-Teller (BET) and Cr (III) adsorption verified their property accordant as ion adsorbents. The effect of mixed acids on species with respect to iron was also investigated by UV-vis spectra. The technique would be easily scaled up for size controllable and property accordant FePO4 nanopartides preparation.