Abstract: During a random vibration test in the y-direction on a certain car drive motor, the DC copper bar inside the motor controller broke. The reasons for the fracture of DC copper bars were analyzed using macroscopic observation, scanning electron microscopy and energy spectrum analysis, metallographic examination, chemical composition analysis, hardness testing. The results show that the fracture mode of the DC copper bar was fatigue fracture. There was stress concentration at the groove defect in the bending R angle of the DC copper bar. Under the action of y-direction load, microcracks were generated at this location, and the cracks continue to propagated, ultimately led to the fracture of the DC copper bar. The cross-sectional area of the DC copper bar at the groove defect decreased, and the resistance at this location increased during conduction, led to an increase in local temperature and a decrease in the mechanical properties of the material, further promoted the initiation and propagation of cracks.