Abstract: To reduce the probability of lamination defect formed during plate steel rolling and improve its quality and mechanical properties specimens of the cold rolled steel plates were nondestructively detected. Then, according to the positions of severe lamination defect found from spectural patterns as the result of detection, the defect, morphologies and compositions of the specimens were analyzed by SEM, etc.. The results showed that the microstructure of lamination defects formed in steel plates are mainly the component of strip/plate sulfides in ferrite bandings distributed along the rolling direction, which is typically the reason why the lamination defects formed. It was concluded that lamination defects can be prevented efficiently if the sulfur content in steel is reduced to below 0.02% and the critical size of sulfides is less than 5 μm.