Abstract: The causes of red rust defects of oxide scale on the surface of DP800 dual-phase steel hot rolled plate were analyzed. The results show that the thickness of the oxide scale in the red rust area on hot rolled plate surface was twice as thick as that of the normal area. The content of Mn, Cr and Mo in the oxide scale of red rust area was low, and the dense oxide scale could not be formed, so the oxide scale in red rust area was loose. After pickling and cold rolling, the chemical compositions of the suface of the red rust area and the normal area were almost the same, the roughness of the red rust area was higher than that of the normal area, and the oxide scale in the red rust area could be completely pickling.