Abstract: 5A06 aluminum alloy forging was found to have linear defect in the machining process. In order to find out the failure cause, morphological observation, metallographic structure examination, fracture surface analysis, hardness analysis and chemical composition analysis were carried out. The result shows that the linear defect is folding defect. It destroyed the continuity of metal and reduced the load capacity of the aluminum alloy forging. The defect may be caused by the inappropriate control in the forging process. It formed during the process of punching and ring forging. In the process of delivery inspection and acceptance test, the forging with the linear defect can be eliminated effectively by adding the ultrasonic inspection and fluoroscope.