A flexible plastic substrate includes a film including a poly(amide-imide) copolymer; and a hard coating layer disposed on a first surface of the film, wherein the hard coating layer includes a crosslinked siloxane copolymer, and wherein the flexible plastic substrate has pencil hardness of greater than or equal to 2H under a vertical load of 1 kilogram according to ASTM D3363, a total light transmittance of greater than or equal to about 89% in a wavelength region of 350 nanometers to 750 nanometers.