The characteristics of poly vinyl alcohol (PVA) and cross-linked PVA were analyzed in terms of the leakage current, the dielectric constant, and the immunity against moisture. For a crosslinked PVA, the leakage current at the same electric field was two orders of magnitude smaller than it was for PVA. Furthermore, the dielectric constant of cross-linked PVA was a little smaller and was less susceptible to changes in the frequency than that of PVA. However, organic thin film transistors (OTFTs) with cross-linked PVA show unsaturated output characteristics even at a drain bias of −50 V. Inserting a poly-methylmethacrylate (PMMA) buffer layer overcame the unsaturated output characteristics of OTFTs with cross-linked PVA gate insulators. In addition, the on-off current ratio, the subthreshold slope, and the threshold voltage were noticeably improved after the buffer-layer insertion.