Interface structure and adhesion of the Ti films on Si substrates pretreated by an Ar ion bombardment have been investigated by high resolution transmission electron microscopy, energy-dispersive X-ray spectroscopy, electron diffraction and Auger electron spectroscopy (AES). Two extra layers were observed between the Ti layer and Si substrate in the as-deposited condition. One was an amorphous Si (a-Si) layer about 2 nm thick which contain Ar atoms on the single-crystal Si surface, and the other is an amorphous Ti-Si (a-Ti-Si) mixed layer about 3 nm thick on the a-Si layer. A peeling test indicates that complete detachment occurred at the interface between the a-Si and the a-Ti-Si mixed layer. However, the adhesion was increased by the heat treatment at 723 K for 30 min, and peeling ratio was reduced to about 10%. Ar atoms distributed at the interface seem to cause the reduction of the adhesion. The heat treatment changed the distribution of Ar atoms at the interface. The profile of the interface was also changed to increase the area of direct contact between the Ti-Si mixed layer and the Si substrate. Both effects seem to enhance the adhesion.