Abstract: In this paper,the widening rule of the deformation layer has been studied during friction welding of a 0.45% carbon steel.When friction is first started,a primitive deformation layer is formed on the friction local interface at a distance of (1/2~1/3) r(radius of the rod) from the center cf the welding cross section.A circular hot ring consists of metallic interface of deformation layer and mechanical draging.As friction is continued,a circular hot ring widens toward both perimeter and center of the welding cross section.High temperature area is widened provided only all friction-al interface has been heated up.A peak power or peak torque is often attained when the heated frictional interface area is increased up to 60-70% of the welding cross section.As all frictional interface has been heated,frictional torque reaches to a constant value.The location,area,thickness,widening direction and widening velocity of the circular hot ring are depended on the parameter of the friction welding.When the peak torque is occured,temperature at the circular hot ring reaches about 700℃.Once the torque value has kept constant,then the temperature at the friction interface may reach 1000℃ or more.