The magnetron sputtering was used to deposit Ni buffer layers on the polyimide surfaces to increase the adhesion strength between Cu thin films and polyimide as well as to prevent Cu diffusion into the polyimide. The Ni layer thickness was varied from 100 to $400{AA}$. The adhesion strength increased rather significantly up to $200{AA}$ of Ni thickness, however, there was no significant increase in strength over $200{AA}$. The XPS analysis revealed that Ni thin films could increase the adhesion strength by reacting with the polar C=O bonds on the polyimide surface and also it could prevent Cu diffusion into the polyimide. The Cu/Ni/ polyimide multilayer thin films showed a high stability even at the high heating temperature of $200^{circ}C$, however, at the temperature of $300^{circ}C$, Cu diffused through the Ni buffer layer into polyimide, resulting in the drastic decrease in adhesion strength.