Abstract: The linear friction welding (LFW) test of TC21 damage-tolerant titanium alloy was carried out.The microstructure characteristics and formation mechanism of the joint were analyzed by using an optical microscope and a scanning electron microscope, the microhardness distribution of the joint was tested by using a microhardness tester.The results show that the TC21 titanium alloy has a high resistance to deformation due to its basketweave microstructure, and a special joint microstructure is formed, which is quite different from that of the LFW joint of duplex titanium alloys.The width of thermo-mechanically affected zone (TMAZ) is very narrow, about 450 μm, and the TMAZ near the weld zone(WZ) has elongated rod α along the friction direction, showing a streamlined appearance.In addition, an obvious heat-affected zone (HAZ) is formed, in which the primary secondary α is decomposed under the thermal effect.The microstructure of TMAZ is deformed to varying degrees due to the gradient heat-force, the primary secondary α is decomposed, and the secondary α gradually precipitates and grows.The dynamic recrystallization has occurred in WZ and fine recrystallized grains are formed, and a large amount of needle α' and fine secondary α are precipitated in recrystallized grains.From BM, HAZ, TMAZ to WZ, the microhardness decreases firstly and then increases, the joint overall shows a "W" shaped distribution.