Abstract: The appearance,microstructures and compositions of iron oxide scale on the strip steel surface were studied by means of SEM,EDS and XRD,and the causes of scale defects were analyzed.The results show that there are four main kinds of scale defects on the surface:①primary scale formed during roughing process which has not been removed by high-pressure water descaling box after slab is discharged from the furnace;②press-in of secondary scale remaining from high-pressure water descaling box before finishing process;③press-in of tertiary oxide scale formed on the surface of hot rolled steel coil due to wear of oxide film on roll surface during finishing and the subsequent cooling processes;④formation of red scale especially on the steel with high silicon content due to strong engagement between surface oxides and base metal during slab heating process.