Abstract: The effective non-destructive testing techniques were studied to test delaminated defect caused during line-cutting in thin-plate with thickness 1.6 to 3 mm. The results showed that the delaminated defect in thin plate might not be detected if only Lamb wave was used, but it was easy to detect delaminated defect with high frequency immersion ultrasonic. Combining the machining technology with the metallurgical analysis, the defect cause was analyzed. The results showed that poor quality material and hot stress from line-cutting were the main reasons causing delaminated defect.