High permittivity polyimide/carbon nanotubes (PI/MWCNTs) nanocomposites with good mechanical flexibility were prepared by electrospinning and hot-pressing. The homogeneity of MWCNTs in PI nanofibers and matrix were confirmed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The PI/MWCNTs nanocomposites showed high dielectric permittivity of 147-161 (at 1 kHz) at percolation threshold of 12-14 vol% concentration of MWCNTs and low dielectric loss below the percolation threshold. The PI/MWCNTs nanocomposites with 20 vol% concentration of MWCNTs had the highest dielectric permittivity of 217 (at 1 kHz), which was 57.5 times higher than the pure PI had. Compared to the maximum energy storage density of pure PI (0.404 J cm(3)), the best maximum energy storage density of 1.957 J/cm(3) was achieved by incorporating 12 vol% of MWCNTs into PI. The PI/MWCNTs nanocomposites (14 vol%) showed good mechanical flexibility with tensile strength, elongation at break and toughness more than 90 MPa, 12.8% and 5.0 J/g respectively and excellent thermal stability with 5% weight loss temperature of more than 560 degrees C. Therefore, this high performance PI/CNTs nanocomposites would be good candidates for modern microelectronics in high temperature areas. (C) 2014 Elsevier Ltd. All rights reserved.