An organic insulator composition according to example embodiments may include an organic insulating polymer and an epoxy-based crosslinking agent. The epoxy-based crosslinking agent may have an alkyl group or a fluorine-based side chain. The organic insulator composition may be used to form an organic insulating layer having increased chemical resistance. The organic insulating layer may be used in an organic thin film transistor as a gate insulating layer. Consequently, the occurrence of hysteresis may be reduced or prevented during the operation of the organic thin film transistor, thus resulting in relatively homogeneous properties.