The magnetic properties of Fe-Ni alloy nanoparticles with particle size in the range 35-45 nm were prepared by almost simultaneously reducing Fe(II) and Ni(II) solution using hydrazine hydrate as a reducing agent in strong alkaline media for two hours at 80 °C. Chemical composition, crystal structure, morphology, thermal stability and magnetic properties of as synthesized Fe-Ni alloy nanoparticles were systematically characterized by means of XRD, TEM, TG-DSC and VSM. These results indicate that there is a vitally important relationship among particle size, particle morphology, and different mol ratio of FeSO4 to NiSO4. The saturation magnetization (Ms). and coercivity (Hc) strongly affected by the composition of Fe-Ni alloy nanoparticles. The hydrothermal reaction is a simple, effective, and low-cost synthetic method to prepare FeNi3 alloy nanoparticles.