A thin film transistor substrate includes an insulating substrate, a gate electrode formed on the insulating substrate, a first gate insulating film formed on the gate electrode and having an opening for exposing at least part of the gate electrode, a second gate insulating film covering the gate electrode exposed by the opening and having a larger dielectric constant than the first gate insulating film, a source electrode and a drain electrode disposed apart from each other in a central area of the second gate insulating film and defining a channel region there between, and an organic semiconductor layer formed in the channel region. A method for forming the TFT substrate is also provided. Thus, the present invention provides a TFT substrate in which a characteristic of a TFT is improved.