The present study applied friction stir welding (FSW) to Ti-15V-3Al-3Cr-3Sn alloy plate, 3 mm in thickness, and the defect-free weld was successfully produced. The microstructure and mechanical properties of the weld were examined. The weld could be classified into four regions, i.e. the stir zone, the thermo-mechanically affected zone, the heat affected zone and the unaffected base material region. The base material consisted of coarse β grains containing lath-shaped substructure with a high density of dislocations. The stir zone exhibited a finer grain structure with a lower density of dislocations than the base material, and the thermo-mechanically affected zone was characterized by the heavily deformed grains. No evident grain growth could be detected in the heat affected zone by optical microscopy. Vickers hardness measurements showed that the stir zone had the lower hardness than the base material, although the finer grains were observed in the stir zone. Transmission electron microscopy revealed that FSW resulted in both destruction of lath-shaped substructure and reduction of dislocation density in the stir zone, which could be qualitatively correlated with hardness profile in the weld.