Abstract: At the joint formed by friction welding of dissimilar metals exists a "bonding zone" of a certain width. In this zone the "branches" grown up from the two dissimilar base metals are interleaved, forming thus an imbedded space-layer structure. The joint is given therefore with the nature of a "body bonding" instead of a simple "surface bonding", and so also superior characteristics of which a joint of "surface bonding" is not possessing. The causes of the formation of space-layer structure are:1. The shifting of the highest temperature surface and the main frictional beat generating surface away from the contact surface due to the difference in the thermo physical properties of the two metals.2. The deformation of incidental rise and fall on the contact surface in the process of rotational rubbing under pressure. Evidences show that this phenomenon takes place in welding of similar metals as well.