Provided is a method for fabricating an organic semiconductor transistor having an organic polymeric gate insulating layer. The method includes forming an organic gate insulating layer on a substrate by a vapor deposition method using organic monomer sources, and causing a polymerization reaction to occur in the organic gate insulating layer to complete an organic polymeric gate insulating layer. Since the vapor deposition method, which is a low-temperature dry-type technique, is employed, the organic polymeric gate insulating layer can be uniformly formed on a large-area substrate by a simplified in-situ process.