In order to improve the interfacial adhesion between polyethylene terephthalate (PET) and metallic copper (Cu), the surface of PET films was modified by treatment with hydrazine monohydrate. The effect of treatment time within 1~20 min with 80 wt% hydrazine monohydrate at 6$0^{circ}C$ and 8$0^{circ}C$, respectively, on the polar group content was investigated. The texture and contact angle of PET film surface, the adhesion strength of PET film and adhesion strength of vacuum-deposited thin copper metal film on the PET film surface were also investigated. The introduction of polar groups on the modified PET folm surface was verified. The amount of polar groups increased up to the maximum value with increasing treatment time up to 10 min at 6$0^{circ}C$ and 3 min at 8$0^{circ}C$, and decreased markedly thereafter. The surface roughness increased with increasing treatment time up to 10 min and cracks occurred after 20 min. Water contact angle and tensile properties decreased with increasing treatment time. Using scratch test, the adhesion between Cu film and PET was found to increase with increasing treatment time up to 10 min, and thereafter there was remarkable decrease in adhesion. From these results, it was concluded that the optimum treatment time of hydrazine monohydrate (80 wt%) at 6$0^{circ}C$ and 8$0^{circ}C$ were about 10 min and 3 min to improve adhesion of copper/PET, respectively. However, the treatment time of 10 min at 6$0^{circ}C$ was observed to be more effective than that of 3 min at 8$0^{circ}C$ to incorporate polar group on the PET surface and to improve adhesion between metallic Cu and PET.