Friction stir welding of TC11 titanium alloy was carried out under different rotational speeds, the microstructures and mechanical properties of the welded joints were analyzed, and the relationships among processing parameters, microstructures and properties of the joints were established. The SZ produced the fully β transformed microstructure, which contained grain boundary α phase, intragranular lamellar α+β phases and acicular α′ phase. The double strengthening was caused by the fine α phase and α+β phases as well as the presence of α′ phase, which led to the highest hardness value in the SZ. The retained original α phase and β transformed microstructure were found in the HAZ and TMAZ, while the TMAZ showed the streamlined deformation characteristics. The proportion of β transformed microstructure in these zones was higher when closer to the SZ, and thus the strengthening effect was enhanced. The size of α phase and α+β phases increased and the content of α′ phase decreased with the rotational speed ascending, which resulted in the reduction of hardness value in the SZ. Similar to the law reflected from the hardness test, all the joints fractured in BM after tensile test, the tensile strengths of the SZs under different rotational speeds were significantly higher than BM, and they decreased with the ascent of rotational speed. Besides, the fine α phase and α+β phases caused the higher elongations of SZs compared with the BM, meaning that the SZ had the good plasticity. With the rotational speed ascending, the elongations of SZs decreased owing to the increase of the size of α phase and α+β phases.