A series of copolyimide/SiO2 hollow sphere thin films were prepared successfully based on bis[3,5-dimethyl-4-(4-aminophenoxy)phenyl]methane and 9,9-bis(4-(4aminophenoxy)phenyl)fluorene (molar ratio = 3 : 1) as,diamine, and 4,4'-(4,4'-isopropylidenediphenoxy)bis(phthalic anhydride) as dianhydride, with different wt % SiO2 hollow sphere powder with particle size 500 nm. Some films possessed excellent dielectric properties, with ultralow dielectric constants of 1.8 at 1 MHz. The structures and properties of the thin films were measured with Fourier transform infrared spectra, scanning electron microscope, thermogravimetric analysis, and dynamic mechanical thermal analysis. The polyimide (PI) films exhibited glass-transition temperatures in the range of 209- 273 degrees C and possessed initial thermal decomposition temperature reaching up to 413-477 degrees C in air and 418-472 degrees C in nitrogen. Meanwhile, the composite films were also exhibited good mechanical properties. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 1133-1137, 2011