Electrical stability of a polyimide siloxane (PSI) film for ultra-large scale integrated circuit (ULSI) multilevel interconnections is studied. The PSI films, modified by p-aminophenyltrimethoxysilane (APTMS), are designed to have three-dimensional polymer structures through Si-O bonds. It has been revealed that the PSI films are more stable in electrical properties at higher temperatures than 150 degrees C, as compared to the conventional polyimide (PI) films. The electrical conduction mechanism study for the PSI films has revealed that Schottky emission is dominant. Barrier height phi(B) obtained from the electrical property for the PSI film was 0.460 eV in the temperatures ranging from 25-250 degrees C, On the other hand, barrier height of 0.422 eV at lower temperatures than 150 degrees C and activation energy of 1.09 eV at higher temperatures than 150 degrees C were obtained for the conventional PI film. The difference in polymer structure is very sensitive to the electrical conduction at high temperature, due to sodium ion migration. The ideal band diagrams of metal-insulator-semiconductor (MIS) structures were also discussed. The optical band gaps for PSI and conventional PI films were 3.320 eV and 3.228 eV, respectively. This result suggests that the band gap of PI films can be enlarged by modification with Si-O components. The differential barrier height between the PSI and conventional PI films is 0.038 eV. and is close to the difference in half of optical band gaps (0.046 eV).