On the basis of our previous work [1] on commercially pure aluminum, the change of internal friction peak of grain bound- (Ke peak) in commercially pure copper has been studied in the course of creep. Creep experiments were conducted under different conditions: (a) high temperature and low stress; (b) medium temperature and medium stress; (c) low temperature and high stress; which lead to intergranular, mixed type and transcrystalline rupture respective'y. From the changes of grain boundary peak under different creep conditions, it can be deduced that the strength of grain boundaries is changed little during creep at high temperature and low stress; somewhat increased at medium temperature and medium stress; and considerably increased at low temperature and high stress. The present work indicates that the strength of grain boundaries is changeable during creep. The extent of strengthening of grain boundaries is different under different creep conditions, which is believed to be an important roie to lead to different type of creep rupture. This provides a modification and complement of the idea of "equicohesive temperature".