In order to clarify the bonding mechanism by plastic deformation of two similar materials, the microstructural evolution of the copper–copper interface by hot compression was investigated in detail. Particular attention was given to the development of new recrystallized grains in the bonding area and the elimination of the bonding line by the migration of interfacial grain boundaries (IGBs) during the hot compression process. The evolution of new grains in the matrix and the interface occur via different types of dynamic recrystallization (DRX). The former is associated with the grain boundary bulging and the latter is related to a strain induced substructure. This substructure is composed of a dislocation wall, and is transformed into recrystallized grain by the evolution of high angular misorientations after increasing strain. Two types of interfaces with different grain size features, such as a fine–fine grain or a fine–coarse grain interface, will be generated owing to different degrees of recrystallization occurring at the interface. The bonding mechanisms of the two different structural interfaces are also discussed.