Ultrafine amorphous Fe-P alloy particles were directly synthesized by the plasma using cyclopentenyl iron and phosphorous trichloride as starting materials. The plasma gas greatly influenced the morphology, dispersion and composition. The particles were roughly spherical with a diameter of 40-200 nm and had the composition of Fe106P50. Elemental chlorine was found in the surface of the particle especially prepared under argon plasma conditions. It was bonded with phosphorus and carbon in Fe-P particles prepared in argon plasma and mainly with carbon in Fe-P particles deposited under a hydrogen plasma. Formation of Fe-P improves the stability of phosphorus in air. Phosphorus en rich ment in the surface of Fe-P particles was also found. The particles were characterized by TEM, SEM, infrared-spectroscopy, X-ray photoelectron spectroscopy quantitative analysis, differential scanning colorimetry, inductively coupled plasma, X-ray diffraction and X-ray microprobe analysis. The formation mechanism of Fe-P amorphous particles was also discussed.