Ti-6Al-4V alloy has been nitrided in low pressure inductively coupled r.f. plasmas of nitrogen, hydrogen, and argon. The structure and composition of the nitrided layers were investigated by a variety of methods. Mechanical properties, i.e. surface hardness, erosion resistance and fatigue of nitrided samples, were studied as a function of preparation conditions. Surface microhardness as high as 29.4 GPA was obtained for the nitrided layers with significant improvements in the fatigue life and erosion resistance of the nitrided alloy.