The implantation of N ion into alpha-Fe surface was carried out by plasma-based ion implantation method at the elevated temperature of 360 degreesC and different implantation voltages of 15 and 35 kV. The phase composition and the surface roughness were studied by means of grazing incidence angle X-ray diffraction and atomic force microscopy, respectively. The results indicated that the implantation voltage has great effects on the iron nitride formation and surface roughness of the implanted layer. The lower the voltage, the greater is the nitrides in the layer, and the higher is the roughness of the surface. The influence mechanisms of implantation voltage on the phase composition of the layer and the roughness of the surface were also discussed. (C) 2002 Elsevier Science B.V. All rights reserved.