Abstract: After running for a period of time, speckled defects with strong regularity were formed on transmission gears of high-speed trains. The speckled defects were located at root of the large cylindrical gear and at top of the small bevel gear. Causes of the speckled defects were analyzed by macroscopic observation, chemical composition analysis, microhardness test, microstructure and microscopic observation. The results show that the speckled defects on the transmission gears were caused by electrical erosion. Due to poor insulation of the train circuit and insufficient grounding insulation of the rails, some stray currents broke through the thin oil film between the tooth surfaces and caused spark discharge, leading to electric erosion. When the degree of the electric corrosion was light, the oil film was carbonized; when the degree was high, local melting was produced and then caused melting beads. Thus the speckled defects were formed.