A surface treatment method of an FRP substrate is provided. According to the method, an FRP substrate that is less deformed than a conventional flexible substrate material is used, and surface roughness, which is a drawback of the FRP substrate, is improved so that it may be applied to the fabrication of a device. In order to reduce the surface roughness of an FRP substrate, the FRP substrate is coated with an organic insulating solution and thereby planarized. Due to the reduced deformation and superior flatness, failures occurring due to misalignment in a photolithography process may be prevented.