We present a simple and easy method to control the size of FeNi3 nanoplatelets by solution reduction reaction. The size of FeNi3 nanoplatelets can be controlled by varying the concentrations of the reactants. Particle structure was studied by means of transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The synthesized single-phased FeNi3 particles are plate-like in shape and with a face-centered cubic crystal structure. The oxidation layer has been found on the surface of the nanoplatelets. The ferromagnetic properties are demonstrated by vibrating sample magnetometer (VSM) studies. The lattice constant and coercivity of the samples are size-dependent. A possible mechanism of the shape control was discussed. (c) 2008 Elsevier B.V. All rights reserved.