Abstract: Microhardness and element distribution in the welding interface of Ti-24Al-15Nb-1.5Mo/TC11 dissimilar alloys was investigated through thermal exposure experiment at 550℃ for 50 h. The results showed that the microhardness of Ti-24Al-15Nb-1.5Mo alloy after gradient heat treatment was higher than that after double heat treatment. Under gradient heat treatment, the decrease of acicular α phases in weld seam with 30% deformation resulted in increase of the microhardness, while the microhardness in Ti-24Al-15Nb-1.5Mo alloy decreased due to the release of lattice distortion energy. However, the microhardess in the welded joints under 40%, 50% deformation and gradient heat treatment decreased after thermal exposure because of the decomposing and growing of phases in the seam. Elements in the welding interface continued diffusing during thermal exposure experiment. The lattice distortion increased as Nb content increased significantly in the weld seam after thermal exposure, which also resulted in the increase of the microhardness.