Abstract: The effect of thickness of Cr buffer layer, Co magnetic layer and Cunonmagnetic layer on the giant magnetoresistance （GMR） in Co/Cumultilayers fabricated by UHV electron beam evaporation was studied. It has been shown that optimized thickness of the Cr buffer layer is necessary to obtain high GMR value. But very thick Cr buffer layer would result in a reduction of GMR value due to its shunting effect. By optimization of the thickness of Co and Cu layers, a GMR value as large as 7% was found in a single Co/Cu/Co sandwich structure. By increasing stacking number to 6, a maximum GMR value of 11% at room temperature has been obtained with a saturation field of 250 Oe in Co/Cu multilayers, which is thermally stable up to 400℃.