The observation on emitting dislocations from grain boundaries by TEM during Cu elongation has been pefformed. It is shown that there exists the “ledge” at the grain boundaries in fcc pure Cu, which is able to emit dislocations into grain under action of stress. The observation on emitting dislocations from grain boundaries by TEM during Cu elongation has been pefformed. It is shown that there exists the “ledge” of the grain boundaries in fcc pure Cu, which is able to emit dislocations into grain under action of stress .