Abstract: The La-aluminized coating on 20-carbon-steel was prepared by hot dip aluminizing method.The microstructure of Al₄C₃ phase in the coating was investigated through transmission electron microscope (TEM)after diffusion treatment at 850℃ for 4 h.The results indicate that the Al₄C₃ phases are rod-shaped and approximately sphericized, and the twinned Al₄C₃ phases exist in the coating.By using conversion matrix of twin index and the conversion matrix acting on twin index, it is proved that the Al₄C₃ phases are 180° secondary rotation twin with (003)as twinning plane and 001 direction as twinning axis.The long axis direction of rod-shaped Al₄C₃ phases is parallel to (003)plane and perpendicular to 001 direction, and its long axis direction is 210 direction.This shows that there is 180°secondary rotation twin with (003) as twinning plane and 001 direction as twinning axis in hexagonal system.The formation mechanism of the twinned Al₄C₃ phases is also discussed in the paper.