What determines the propagation of the surface layer if nitriding parameters or alloy composition were changed? By adjusting nitriding temperature and nitrogen partial pressure. the surface layers grown on titanium and Ti-6Al-IV were characterized in this study to reveal the propagation of each layer. Ion nitriding was carried out in a d.c. glow discharge. The nitrided specimens were characterized by a number of techniques including optical microscopy, scanning electron microscopy, wavelength dispersive spectroscopy and X-ray diffraction. Surface microhardness measurements were conducted in order to evaluate the effect of the processing parameters on the surface layers. The results show that the aluminum segregated layer forming between the compound la-er and diffusion layer hinders the diffusion of nitrogen toward the con of Ti-6Al-4V and reduces the growth rate of the compound layer without affecting diffusion layer thickness. The change in plasma conditions such as H-2 gas concentration can only have the effect on a nitrided compound layer, The most efficient ionization occurs at the H-2 gas concentration of 17.5% in our case, The activation energy measured is lower than those obtained at higher nitriding temperature. This is believed to be the result of the changes in diffusion path that are affected by the compound layer texture and internal stress.