Abstract: A cubic boron nitride(c-BN) surface hardened film of 0.15~0.2mm was prepared by a new plasma immersion ion implantation(PIII) technology on the specimens of boronised 50Mn steel.Cubic boron nitride(c-BN),hexagonal boron nitride(h-BN),B₂O₃,FeB and Fe₂B were found in the hardened film under X-ray photoelectron spectroscopy(XPS) and X-ray diffraction (XRD).The film contents rich c-BN in the region from the surface to the depth of 60 nm.A sliding friction test in unlubricated condition and a micro Vickers hardness test were conducted to compare the properties of the specimens treated by boronising plus PIII and by boronising alone.The results of the tests indicated that the specimens of 50Mn steel treated by boronising plus PIII has far higher(up to HV_0.1N44GPa) hardness and wear resistance than the specimens of 50Mn steel boronised alone,and an application practice of the c-BN film in the moulds of telegraph-cable reached a satisfactory effect.