Microstructure and composition of diffusion layer of the hot dip aluminized steel after diffusion at 850 ℃ for 4 hours were investigated by TEM （transmission electron microscope）. The results show that spinodal decomposition phenomenon is observed in Fe2Al5 and FeAl2 phases in diffusion layer. The pattern of spinodal decomposition structure is involved from parallel beeline to parallel across striation and then to lattice shape from the surface to inner of diffusion layer. There exist obvious effects of grain boundary on spinodal decomposition structure in some area of diffusion layer. In addition

there exist different spinodal decomposition structures in every grain. The stripes of the spinodal decomposition structure exhibit lamellar structure in inner in whole crystal. The formation mechanism of the spinodal decomposition structure was described based on the 15% rule.