Owing to the rapid increase in the demand for miniaturized flexible circuit devices, such as flexible copper clad laminate (FCCL), the development of a polymer substrate capable of adhering to the Cu film without deterioration of the dielectric properties is becoming a challenging issue. This study presents a process for forming a robust Cu adhesive layer on poly(ether ether ketone) (PEEK) while preserving the thermal, mechanical, and dielectric properties of PEEK. By simple UV irradiation without any precise control, the benzophenone group in PEEK chain structure can produce free-radicals (self UV-initiated polymerization). Using this principle, 2,3-epoxypropyl methacrylate and 3-tri(methoxysilyl)propyl methacrylate were copolymerized on the PEEK surface (PEEK-g-Polymer). The peel strength of the Cu adhered PEEK-g-Polymer, measured by a 90° peel test, was 9.40 N/cm, which satisfies the required value for FCCLs. The differential scanning calorimetry, tensile, and dielectric tests confirmed that the thermal, mechanical, and dielectric properties of PEEK were preserved after the polymer grafting process, and the dielectric loss of PEEK-g-Polymer was lower than that of commercial polyimide-based substrate. This straightforward approach is expected to make a great contribution to achieving lower dielectric loss and miniaturization for 5G devices.