On-site samples about color difference defect on the surface of hot-rolled low carbon DC04 from steel enterprises were analyzed，and it was determined that the difference of the thickness and the structure of oxide scale between the surfaces in the middle and in the edge was the main reason for the color difference on the surface of hot-rolled strip. The transformation law of microstructure of oxide scale in continuous cooling was simulated and investigated by means of thermogravimetric analysis （TGA） and electron probe microanalysis （EPMA） in order to solve this problem. The results showed that pro-eutectoid Fe3O4 was prior to nucleate at the original Fe3O4 layer. The lamellar eutectoid structure was obtained as the cooling temperature decreased. However， when the cooling rate reached 25 ℃/min， the eutectoid reaction was suppressed in the oxide scale. Industrial trial production was implemented based on the results of laboratory research， the microstructure and thickness of oxide scale were effectively controlled by adjustment of coiling temperature and cooling rate， and the cooling temperature was reduced to the temperature range of eutectoid transformation，and then the color difference defect was basically eliminated.