The magnetron reactive sputtering was adopted to deposit CuO buffer layers on the polyimide surfaces for increasing the adhesion strength between Cu thin films and polyimide, varying $O_2$ gas flow rate from 1 to 5 sccm. The CuO oxide was formed through all the $O_2$ gas flow rates of 1 to 5 sccm, showing the highest value at the 3 sccm $O_2$ gas flow rate. The XPS analysis revealed that the $Cu_2O$ oxide was also formed with a significant ratio during the reactive sputtering. The adhesion strength is mainly dependent on the amount of CuO in the buffer layers, which can react with C-O-C or C-N bonds on the polyimide surfaces. The adhesion strength of the multi-layered Cu/buffer layer/polyimide specimen decreased linearly as the heating temperature increased to $300^{circ}C$, even though there showd no significant change in the chemical state at the polyimide interface. This result is attributed to the decrease in surface roughness of deposited copper oxide on the polyimide, when it is heated.