In order to obtain thin film dielectric layers with very low dielectric constants for use in microelectronic devices, polyimide nanofoamed films has been prepared from the polyimide precursors (PMDA-ODA) and poly(ethylene oxide) (PEO) in N,N-dimethylacetamide. The synthesization process included blending polyimide as the major phase with a minor phase of the thermally labile PEO blocks. The foamed films were characterized by a variety of experiments including TG and SEM, and the experimental results indicated that the labile PEO would undergo oxidative thermolysis to release small moleculars from the matrix so as to leave voids into the polyimide matrix. The dielectric properties of the films were studied over broad frequency ranges. The dependences of dielectric constant on the PEO content in the films and frequency were discussed. The films with a proper amount of PEO displayed relatively low dielectric constant compared to the pure polyimide film. Thin film foams with high thermal stability and low dielectric constants can be prepared using the approach.