A concentric thermoelectric filament structure used for radioisotope thermoelectric generator (RTG) is designed to satisfy low-power and high-voltage demands for aerospace microelectronic devices. The thermoelectric filament is proven to have a better output voltage than the traditional thermoelectric structure by using COMSOL. The unbreakable thermoelectric filament and corresponding devices are prepared by employing a simple and easy brush coating process. A single thermoelectric filament obtains an open-circuit voltage (V-oc) of 6.0 mV and a maximum power output (P-max) of 2.0 mu W at 398.15 K hot surface temperature. An array of thermoelectric filaments is fabricated for the radioisotope heat source of planar heat surface, which obtains V-oc of 83.5 mV and P-max of 32.1 mu W at the hot surface temperature of 398.15 K, thereby verifying the practicability of the concentric filament structure in devices. Another radial thermoelectric filament device is designed for the cylindrical heat source. This device exports 84.5 mV V-oc and 42.5 mu W P-max at the hot surface temperature of 398.15 K. These thermoelectric devices could gain larger electrical performance in small space through further series connection and fabrication, which will be a commendable design scheme for the radioisotope thermoelectric generator.