Orientation-controlled copper bicrystal was deformed in tension at 923 K at a strain rate of 4.2 x 10(-3) s(-1) with the grain boundary aligned parallel to the loading axis and the nucleation behavior of dynamic recrystallization (DRX) along the grain boundary was investigated. After grain-boundary migration (GBM) and bulging, nuclei appeared preferentially at the migrated grain boundary at about a strain of epsilon = 0.15. This strain was about half of the peak strain. All the nucleated grains were twins (Sigma3). Furthermore, all twinning planes were either the 3(rd) or the 4(th) slip plane of the component single crystals. The DRX nucleation mechanism on grain boundary is discussed in relation with GBM and twinning.