Diffusion bonding of Ti and Ti-Al (Ti-10, 20 and 40 mol%Al) alloys to high carbon steel was carried out at 1073 and 1273 K for 3.6 ks in a vacuum. It has been found that the joint with Ti-20 mol%Al alloy is separated in the vicinity of the interface promptly after the bonding treatment at 1273 K. Such a phenomenon could not be observed in other Ti-Al alloy/steel joints, and the Ti-20 mol%Al alloy/steel joint bonded at 1073 K showed a high strength of about 170 MPa. Therefore, this phenomenon depends on the bonding temperature and the composition of the Ti-Al alloys. From the observation results of the interface, it is thought that the diffusion of constituent elements across the interface is part of the reason for the separation phenomenon. The separated surface of the Ti-20 mol%Al alloy side showed diffraction peaks of TiC phase by XRD. Its Vickers hardness was about 1200 and approximately 4 times higher than that before the bonding treatment. It is expected that the separation phenomenon at the Ti-20 mol%Al alloy/steel interface serves surface modification of titanium materials, which show poor wear resistance.