The present paper presents a facile and low-cost hydrothermal method to synthesize stoichiometric FeNi3 alloy nanoparticles by reducing Ni(NO3)2.6H(2)O and Fe(NO3)2.9H(2)O with hydrazine hydrate in strong alkaline media. X-ray diffraction patterns of as-prepared samples synthesized at 180 degrees C for different hydrothermal reaction times reveal that the products are pure stoichiometric FeNi3 alloyed nanoparticles with well-defined crystalline cubic structure when the hydrothermal time is above 2 h, while no crystal phases are detected in the product obtained under ambient pressure in an open system from the same starting prescription. Transmission electron microscopic images show that the size of the as-prepared nanoparticles increases with prolongation of the hydrothermal reaction time. The as-prepared sample has the symmetric hysteresis loop behavior of ferromagnetic materials.