Ti-6Al-4V alloys were nitrided through gas nitriding at 950 °C for 3 h in deoxygenated, atmospheric nitrogen gas. During nitriding, nitrogen reacted and diffused into the alloys to form Ti2N and a meager amount of TiN in a Ti-N compound layer with a thickness of 20 μm to 25 μm. An α-Ti(N) diffusion layer with a thickness of 40 μm to 80 μm formed below this layer. A small amount of Al was dissolved at the top of the Ti-N compound layer because of the strong interaction of nitrogen with Ti and Al. Nitriding resulted in the dissolution of interstitial nitrogen and the formation of nitrides. Oxidation of the nitrided Ti-6Al-4V alloys initially resulted in the formation of a Ti-N-O layer, which later oxidized to TiO2. Above 800 °C, the nitrided alloys oxidized rapidly, accompanied by microcracking of the TiO2 surface layer.