The air-stability of n-channel organic thin film transistors (OTFTs) has been improved by modifying the dielectric surfaces with polymer insulators. The OTFTs were fabricated using N,N'-dioctyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C8) as the active material and SiO2 modified with different polymers as the gate dielectric. Among the polymer insulators, the device modified with hydroxyl-free polymers exhibited the best performance. For example, the electron mobility, 0.11 cm2 /Vs, in the saturation region, was obtained in the ambient condition for the device with the SiO 2 surface modified with the polymers. The on-off ratio was also as high as 105 . More importantly, the devices modified with hydroxyl-free polymers had better air-stability. The improved stability could be attributed to the passivation of the hydroxyl groups (electron trapping sites) on the SiO2 surface.