Abstract: The hairline crack defects of hot rolling plate and galvanized steel sheet made in Benxi Iron &Steel (Group) Co. Ltd is studied through microscope and SEM in this paper. The metal graphic tests about the marostructure and microstructure of defects. The defects position and the changing of metallurgical quality are made in this paper. Basing on the above tests, we think that the hairline crack is induced by the remained deep vibration mark of casting slab.