Abstract: The display of ferrite grains in ultra-low carbon steel was compared and analyzed by metallographic method and ion milling technology. The results show that only a small amount of ferrite grain interface was corroded by conventional 4% nitric acid alcohol solution, and the microstructure display effect was poor. The grain boundary of ferrite can be clearly displayed by etching the sample with a special copper sulfate solution, but the etching parameters during sample preparation were not easy to control, so the etching parameters need to be adjusted repeatedly and repeatedly, and the success rate was not high. Ferrite grains can be clearly displayed by ion milling technology, and the corresponding key optimal parameters of ion milling are given, including the deflection angle α is 30° and grinding time is 15 min. The other parameters are the acceleration voltage of 5.5 kV, the output voltage of 1.5 kV, the gas flow rate is 0.09 cm³·min^-1, the sample holder speed is 25 r·min^-1, and the speed mode is 360° rotation.