Provided are a composition for an organic polymer gate insulating layer and an Organic Thin Film Transistor (OTFT) using the same. The composition includes an insulating organic polymer including at least one selected from the group consisting of polymethylmethacrylate (PMMA), polyvinylalcohol (PVA), polyvinylpyrrolidone (PVP), poly(vinyl phenol) (PVPh) and a copolymer thereof, a crosslinking monomer having two or more double bonds, and a photoinitiator. The OTFT includes a gate insulating layer of a semi-interpenetrating polymer network formed of the composition. The composition for a photoreactive organic polymer gate insulating layer has a photochemical characteristic that enables micropatterning, and can be formed into a layer having excellent chemical resistance, thermal resistance, surface characteristics and electrical characteristics.