Al 2O 3/SiN x double-layered dielectric films suitable for large-size amorphous silicon thin film transistor liquid crystal displays (a-Si:H TFT LCD) have been prepared by anodization and plasma enhanced chemical vapor deposition (PECVD). Al 2O 3 films were formed at various pH values and volume ratio of water (H 2O%) in the electrolyte. The optimal quality of Al 2O 3 film was achieved at pH = 6 and H 2O% = 30. These high-quality Al 2O 3 films have denser structure, lower etching rate (90 Å min −1) and smooth surface morphology after etching. The capacitors made of the anodization show a breakdown field as high as 7.8 MV cm −1 and a low leakage current density of 10 nA cm −2 at a dielectric field of 3 MV cm −1. Furthermore, a-Si:H TFTs with Al gate and different gate dielectrics (Al 2O 3/SiN x and SiN x ) were also fabricated and evaluated. The TFTs with Al 2O 3/SiN x double-layered gate dielectric provide better performances, such as the smaller threshold voltage (1.76 V), improved subthreshold swing (1.0 V dec −1), and lower off-current (3.8 PA) than those of the device with single SiN x film.