Solution nitriding and aging treatment were applied to Ti-4mass%Cr alloy in order to fabricate a ductile high-nitrogen titanium alloy with fine (α + β) structure. The solution-nitrided specimen withα’ martensitic structure was significantly hardened by solid solution strengthening by the absorbed nitrogen. During the aging treatment, fine β grains with a size of 1 microns in thickness precipitated along the martensite-plate boundaries. Although the specimen was softened to some extent after the aging treatment, the hardness is kept much higher than that of the aged Ti-4mass%Cr alloy without solution nitriding. This indicates that the nitrogen is still in solid solution of α phase even after the aging treatment, and contributes to strengthening of the fine-structured Ti-4mass%Cr-N alloy.