Thin films of 3-methacryloyloxypropylheptaisobutyl-T8-silsesquioxane (BuPOSSMA) were prepared by an electron-assisted (e-assist) deposition method. The film deposited by the conventional vapor deposition without the e-assist had a polycrystalline structure with a rough surface, whereas the e-assist deposition produced amorphous polymer thin films with a smooth surface. The deposited films were electrically insulating with a breakdown strength higher than $2\times 10^6$ V/cm. The polymerization of BuPOSSMA resulted to decrease the dielectric constant of the film. It was found that the deposition-polymerization achieved by the e-assist is effective in improving thermal stability of the insulating and dielectric characteristics.