Abstract: In this paper the tool wear in milling of titanium alloys, the contact length between tool flank and workpiece and the temperature field on the tool flank during milling operation have been studied. The wear mechanism of cemented carbide tools in milling of titanium alloys has been studied. At low speed milling, the major type of the of the tool wear is the adhesion tear wear on the tool flank. However, at high speed milling, the tool wear is mainly caused by diffusion. The evidence of the diffusion of cobalt and carbon in boundary layer between the milling cutter and the workpiece has been obtained. Furthermore it was first revealed that the carbon rich layer was formed in the boundary layer and carbon poor layar was found in the sub-layer under the tool flank as the diffu sion wear took place.