Abstract: Three cases of high-voltage fault phenomena, cause and repairing method for TecnaiG-20 type 200 kV transmission electron microscope manufactured by FEI were analyzed. The results illustrate that all these three faults came from electron gun, but the causes were different from each other. The first fault was due to the aging of insulating silica gel on surface of the cable head, which caused decline in insulating property and could be repaired by recoating new insulating silicagel method. The second fault was owing to the damage of accelerating pole resistance, which caused a discharge in internal of the electron gun. The fault could be repaired by replacingit with a new part. The third fault arose from the damage of magnetic bottle in internal of the electron gun, which could be repaired by treatment to the magnetic bottle. By analyzing and repairing, all the high-voltage faults were removed and good results were achieved.