Dielectric capacitors are always faced with high temperature in many application cases, so the applicability of high temperature is highly desired for dielectrics besides large energy storage density. Polyimide is a widely used engineering polymer with excellent thermotolerance and ceramic nanofillers with large aspect ratio are effective in improving the dielectric properties of polymer nanocomposite. In this work, PI nanocomposite films contained with BaTiO3 nanofibers prepared via electrospinning were fabricated by solution casting and thermal imidization process. The dielectric properties of the nanocomposites were investigated from room temperature to 200 degrees C. With the introduction of BaTiO3 nanofibers, the dielectric permittivity was enlarged with weak increase on dielectric loss in nanocomposites. The breakdown strength (550 kV/mm) as well as discharged energy density (5.82 J/cm(3)) was largely enhanced in BaTiO3/PI nanocomposite contained with 1 vol% nanofibers. Benefited from the decreased leakage current and improved thermal conduction induced by BaTiO3 nanofibers, the 1 vol% BaTiO3/PI nanocomposite also represented high energy efficiency and excellent thermal stability. The discharged energy density of above 2.1 J/cm(3) and near 4 J/cm(3) with efficiency larger than 90% are stable from room temperature to 150 degrees C and to 100 degrees C respectively. The high temperature applicability of the present material makes it much promising in fabricating dielectric energy storage devices applied in hot environment.