Crystalline FeCo alloys (Fe 64 Co 36 , Fe 59 Co 41 , and Fe 37 Co 63 ,) nanoparticles in diameter of 5–12nm were synthesized by reduction of FeCl 3 and CoCl 2 with hydrazine under ultrasonic wave, which is a simple, low-cost, surfactant-free route, and may stimulate technological interests. As-synthesized Fe 59 Co 41 nanocrystallite shows excellent soft magnetic behavior with high saturation magnetization up to 216.2emu/g that is comparable with that of bulk Fe and Fe 60 Co 40 alloy, and could have applications in catalysis, biotechnology, and magnetic storage devices.