One-pot polyol processing was successfully used for the preparation of FexNi100-x nanoparticles. By increasing Ni concentration in FexNi100-x alloy, the phase transition from bcc to fcc crystalline structure was clearly indicated by the XRD data as well as the measured hysteresis loops. FORC diagrams demonstrated that the prepared samples consist of single-domain magnetic particles with a uniform particle size. By changing the molar ratio of the precursors, different compositions of FeNi alloy were obtained. The influence of the temperature, solvent, surfactant, and amount of NaOH was also studied on the particle size, structure, and magnetic properties of the products.