The aluminized layer and the La-aluminized layer on 20 carbons steel were prepared by hot dip aluminizing method, and the microstructure and composition of the layer were investigated through scanning electronic microscopy (SEM), transmission electron microscope (TEM), X-ray diffractometer (XRD) and energy dispersive spectroscopy (EDS). The results showed that the amorphous phase was found in the surface layer of the hot dip aluminized steel. In order to study the phenomenon, the Al-Fe alloy liquids used in hot dip aluminizing were quickly solidified and slowly solidified respectively. The results showed that the amorphous phase was found in all Al-Fe alloy samples under the condition of the quick solidification. While the amorphous phase was only found in Al-Fe alloy sample with La under the condition of the slow solidification. Moreover, the pure amorphous alloy exists in the parts region of the sample, and the weight percentages of Al and Fe in pure amorphous alloy was 23: 2, and the amorphous alloy composition also contains a small quantity La. The formation mechanisms of the amorphous phase and behavior of La in the hot dip aluminized layer were also discussed.