Polyimide (PI) nanocomposites with different proportions of Al2O3 were prepared via two-step reaction. Silicon nitride (Si3N4) was deposited on PI composite films by a RF magnetron sputtering system and used as a gas barrier to investigate the water vapor transmission rate (WVTR). The thermal stability and mechanical properties of a pure PI film can be improved obviously by adding adequate content of Al2O3. At lower sputtering pressure (4 mTorr), the PI/Al2O3 hybrid film deposited with Si3N4 barrier film exhibits denser structure and lower root mean square (RMS) surface roughness (0.494 nm) as well as performs better in preventing the transmission of water vapor. The lowest WVTR value was obtained from the sample, 4 wt.% Al2O3-PI hybrid film deposited with Si3N4 barrier film with the thickness of 100 nm, before and after bending test. The interface bonding, Al-N and Al-O-Si, was confirmed with the XPS composition-depth profile. (C) 2011 Elsevier B.V. All rights reserved.