A flexible plastic substrate comprising a film comprising a poly(amide-imide) copolymer, and a hard coating layer disposed on one surface of the film including the poly(amide-imide), wherein the hard coating layer comprises a siloxane copolymer cross-linked through a cross-linkable functional group thereof, the plastic substrate has pencil scratch hardness of greater than or equal to 2H measured at a vertical load of 1 kg according to ASTM D3363, transmittance of greater than or equal to 89 % in 350 nm to 750 nm wavelength region; and wherein the siloxane copolymer comprises a curing product of the siloxane copolymer represented by at least one of Chemical Formula 1 to Chemical Formula 3: (Chemical Formula 1) (R 1 SiO 3/2) x (R 2 R 3 SiO 2/2) y wherein, in Chemical Formula 1, R 1 is a cross-linkable functional group or a hydrocarbyl group substituted with a cross-linkable functional group, R 2 and R 3 are each independently hydrogen, a cross-linkable functional group, a hydrocarbyl group substituted with a cross-linkable functional group, a substituted or unsubstituted C1 to C20 alkyl group, or a substituted or unsubstituted C5 to C20 cycloalkyl group, provided that at least one of R 2 and R 3 is a substituted or unsubstituted C1 to C20 alkyl group, and 0<x<1, 0<y<1, and x+y=1, (Chemical Formula 2) (R 1 SiO 3/2) x (R 4 SiO 3/2) z wherein, in Chemical Formula 2, R 1 is a cross-linkable functional group or a hydrocarbyl group substituted with a cross-linkable functional group, R 4 is an ultraviolet absorbing functional group or a C1 to C20 hydrocarbyl group substituted with an ultraviolet absorbing functional group, and 0<x<1, 0<z<1, and x+z=1, (Chemical Formula 3) (R 1 SiO 3/2) x (R 2 R 3 SiO 2/2) y (R 4 SiO 3/2) z wherein, in Chemical Formula 3, R 1 is a cross-linkable functional group or a hydrocarbyl group substituted with a cross-linkable functional group, R 2 and R 3 are each independently hydrogen, a cross-linkable functional group, a hydrocarbyl group substituted with a cross-linkable functional group, a substituted or unsubstituted C1 to C20 alkyl group, or a substituted or unsubstituted C5 to C20 cycloalkyl group, provided that at least one of R 2 and R 3 is a substituted or unsubstituted C1 to C20 alkyl group, R 4 is an ultraviolet absorbing functional group or a C1 to C20 hydrocarbyl group substituted with an ultraviolet absorbing functional group; and 0<x<1, 0<y<1, 0<z<1, and x+y+z=1. R 1 is a cross-linkable functional group or a hydrocarbyl group substituted with a cross-linkable functional group, R 2 and R 3 are each independently hydrogen, a cross-linkable functional group, a hydrocarbyl group substituted with a cross-linkable functional group, a substituted or unsubstituted C1 to C20 alkyl group, or a substituted or unsubstituted C5 to C20 cycloalkyl group, provided that at least one of R 2 and R 3 is a substituted or unsubstituted C1 to C20 alkyl group, and 0<x<1, 0<y<1, and x+y=1, R 1 is a cross-linkable functional group or a hydrocarbyl group substituted with a cross-linkable functional group, R 4 is an ultraviolet absorbing functional group or a C1 to C20 hydrocarbyl group substituted with an ultraviolet absorbing functional group, and 0<x<1, 0<z<1, and x+z=1, R 1 is a cross-linkable functional group or a hydrocarbyl group substituted with a cross-linkable functional group, R 2 and R 3 are each independently hydrogen, a cross-linkable functional group, a hydrocarbyl group substituted with a cross-linkable functional group, a substituted or unsubstituted C1 to C20 alkyl group, or a substituted or unsubstituted C5 to C20 cycloalkyl group, provided that at least one of R 2 and R 3 is a substituted or unsubstituted C1 to C20 alkyl group, R 4 is an ultraviolet absorbing functional group or a C1 to C20 hydrocarbyl group substituted with an ultraviolet absorbing functional group; and 0<x<1, 0<y<1, 0<z<1, and x+y+z=1.