This work was carried out to prevent the rolled-in defects originated from the roll surface deterioration through the investigation of the effect of the thermal fatigue property of hot strip mill work roll materials on the roll surface deterioration and subsequent surface defects in the cold-rolled ultra-low carbon steel strips. The thermal fatigue property was estimated by conducting the thermal fatigue test for an existing nickel-grain iron and a candidate high speed steel in the temperature range of 200 to 600°C, and its result was interpreted on the base of the microstructures, the mechanical and physical properties of the roll materials. It was found that the high speed steel had much better thermal fatigue property than the nickel-grain iron, which was attributed to a lower carbide content, a higher tensile and compressive strength and a higher thermal conductivity. And then, the effect of the roll materials on the rolled-in defects in the ultra-low car-bon steels was investigated by inspecting the work roll surface deterioration in actual hot strip mill and subsequent surface defects in cold-rolled strips. The high speed steel roll with an excellent thermal fatigue property showed much higher resistance against roll surface deterioration than the nickel-grain iron roll, which led to prevention of the surface defects in the cold-rolled ultra-low carbon steel strips.