Abstract: Many pit defects appeared on the tooth surface of gear made of 20CrNi2Mo steel after working for a certain time. The causes of those defects were analyzed by macroscopic analysis, scanning electron microscopic analysis and metallographic examination. The results show that the pit defects on the surface and top of gear teeth were caused by electric corrosion. Local oil film cannot be established due to electric corrosion and martensite with high hardness and brittleness formed after electric erosion discharge, which resulted in gear wear and contact fatigue spalling under the cyclic engagement.