We have used transmission electron microscopy (TEM) and image simulation techniques to systematically investigate the surface microstructures evolved in Ti-6Al-4V alloy after nitriding by intensified plasma assisted processing (IPAP). Crosssectional TEM and X-ray energy-dispersive spectroscopy studies demonstrated that the surface layer in the IPAP-treated Ti-6Al4V has a N composition of approximately 25 at.%. By using electron diffraction analysis, the nitride layer was found to be composed of an ordered superstructure Ti3N(theta-phase) that can be identified as a trigonal structure with a space group of P3 and lattice parameters a = 6.04 Angstrom and c = 4.82 Angstrom (hexagonal setting). The 3(d)-(1/2 0 0) and 3(d)-(1/ 3 1/6 1/2) positions in the unit cell are occupied by metallic (Ti, Al, V) atoms, whereas the 1(a)-(0 0 0) and 1(c)- (1/3 2/3 1/2) positions are occupied by N atoms. The N content at the surface layer gradually decreases with depth, producing a composition gradient structure. This composition gradient layer is accomplished by gradually reducing the amount of Ti3N phase and increasing that of the alpha-Ti structure.