Disclosed is a method of producing a thin film transistor substrate having high light sensitivity, heat-resistance, impact resistance, and a photosensitive composition used by the same, the method including forming data wires on an insulating substrate, forming an organic insulating film on the data wires by applying a photosensitive composition comprising a terpolymer, where the terpolymer is derived from monomers of an unsaturated carboxylic acid, an unsaturated carboxylic acid anhydride, or a mixture thereof, an unsaturated epoxy group-containing compound, and an olefinic compound.