We report the preparation processes and magnetic properties of ε' iron carbide ultrafine particles. Ultrafine particles of δ-FeO(OH) and CoFe₂O₄ were selected as starting materials for ε' iron carbide ultrafine particles. δ-FeO(OH) particles and CoFe₂O₄ particles were coated with SiO₂ for preventing agglomeration of particles during heating. δ-FeO(OH) particles were heated at 450°C for 3h in air to be transformed into a-Fe₂O₃. The α-Fe₂O₃ particles were granular and a little sintered in spite of SiO₂ coating. After α-Fe₂O₃ and CoFe₂O₄ were reduced to metal, α-Fe metal and alloy particles were respectively carburized at 210°C for 4h in the mixing gas flow of CO and H₂ in the volume ratio of 1/4. XRD patterns reveal that both particles are ε' iron carbide single phase. Both of ε' iron carbide phase particles were granular and 20-30nm in diameter. The saturation magnetizations of the ε' iron carbide ultrafine particles obtained from starting materials of δ-FeO(OH) and CoFe₂O₄ were 65 and 69 emu/g respectively, while the coercive forces were 1200 and 1270 Oe.