Abstract: Fe-Al intermetallic matrix composite coating was prepared on the surface of steel C45 by hotdip aluminizing,then the sample was processed at different diffusion annealing temperature. The structures and chemical compositions of the coating were analyzed by XRD and SEM etc. The effect of diffusion annealing temperature on alloy layer and the conversion mechanism of the layer was discussed. The results show that the main phases of the coating are Fe₂Al and Al₂O₃ when the temperature is at 700~800℃; as the temperature exceeds 800℃,the phase of the alloy layer varies from brittle phase(Fe₂Al₅) to tough phases(FeAl and Fe₃Al); the alloy layer is completely converted to FeAl and Fe₃Al when the diffusion temperature is at 1 000℃. Fe₂Al₅ is featured by great hardness but it tends to crack due to its brittleness. Although the hardness of FeAl and Fe₃Al is lower than that of Fe₂A₅,their toughness is so excellent that they can effectively hinder the formation and expansion of cracks in the coating,and they can integrate closer with the matrix.